The stability of a horizontal porous layer of a ferromagnetic fluid heated from below is studied when the fluid layer is subject to a time-periodic body force. Modified Darcy law is used to describe the fluid motion. The effect of gravity modulation is treated by a perturbation expansion in powers of the amplitude of modulation. The stability of the system, characterized by a correction Rayleigh number, is determined as a function of the frequency of modulation, magnetic parameters, and Vadasz number. It is found that subcritical instability is possible for low frequency g-jitter and that the magnetic and g-jitter mechanisms work against each other for small and moderate values of frequency of modulation. The effect of Vadasz number is shown to be reinforcing the influence of gravity modulation for small and moderate values of frequency. The magnetic, porous and modulation effects disappear altogether for sufficiently large values of the frequency of modulation. Keywords-Ferromagnetic fluid, Gravity modulation, Perturbation method, Porous layer, Stability. Gupta and Gupta (3) investigated thermal instability in a layer of ferromagnetic fluid subject to coriolis force and permeated by a vertical magnetic field. It is substantiated that overstability cannot occur if the Prandtl number is greater than unity. Gotoh and Yamada (4) investigated the linear convective instability of a ferromagnetic fluid layer heated from below and confined between two horizontal ferromagnetic boundaries. The Galerkin technique is used and the Legendre polynomials are taken as the trial functions. It is shown that the magnetization of the boundaries and the nonlinearity of fluid magnetization reduce the critical Rayleigh number and the effects of magnetization and buoyancy forces are shown to compensate each other. Blums (5) examined the possibility of having convection in ferromagnetic fluids as a result of magneto- diffusion of colloidal particles which give rise to non-uniform magnetization. Stiles and Kagan (6) examined the thermoconvective instability of a horizontal layer of ferrofluid in a strong vertical magnetic field. Their work also questioned the satisfactory agreement claimed to exist between the experiments and the theoretical model of Finlayson which has been generalized by them. Odenbach (7) investigated the convective flow generated by the interaction of a magnetic field gradient with a gradient in magnetization in a magnetic fluid. This gradient was caused by the diffusion of the magnetic particles in the field gradient. Aniss et al. (8) investigated the effect of a time-sinusoidal magnetic field on the onset of convection in a horizontal magnetic fluid layer heated from above. The Floquet theory is used to determine the convective threshold for free-free and rigid-rigid cases. The possibility to produce a competition between the harmonic and subharmonic modes at the onset of convection is discussed. Abraham (9) investigated the Rayleigh-Benard problem in a micropolar ferromagnetic fluid layer in the presence of a vertical uniform magnetic field analytically. It is shown that the micropolar ferromagnetic fluid layer heated from below is more stable as compared with the classical Newtonian ferromagnetic fluid. The effect of radiative heat transfer on ferroconvection has been studied by Maruthamanikandan (10) using the linear stability analysis. Consideration is given to two asymptotic cases, viz., transparent and opaque layers of fluid. The critical values marking the onset of convection are obtained using the Galerkin technique. Bajaj (11) considered thermosolutal convection in magnetic fluids in the presence of a vertical magnetic field and bifrequency vertical vibrations. The regions of parametric instability have been obtained using the Floquet theory. Ramanathan and Muchikel (12) investigated the effect of temperature-dependent viscosity on ferroconvective instability in a porous medium. It is found that the stationary mode of instability is