The magnetic phase transition is studied by the AC susceptibility and the specific heat measurements in NaNi triacetylacetonate benzene which is considered as an example of an antiferromagnetic triangular Ising lattice. The magnetic phase transition from paramagnetic to ferrimagnetic state is observed at the temperature \(T_{\text{c}}^{0} \cong 32.5\) (mK) by the AC susceptibility measurements. The H vs T phase diagram is obtained giving the extrapolated value of \(H_{\text{c}}^{0} \cong 320\) (Oe) at T =0 K. At T c 0 the susceptibility does not diverge to infinity, and below T c 0 it increases with decreasing temperature. This new type of behavior of the ferrimagnetic transition is qualitatively well described by the antiferromagnetic triangular Ising lattice model.