A rigorous field analysis is carried out to obtain the eigenvalue equation for hybrid modes in a rectangular waveguide periodically loaded with uniform E-plane strips on a dielectric substrate. The eigenvalue equation is in the form of a homogeneous matrix equation for the amplitudes of the spatial harmonics of a field vector defined on the interface of the dielectric substrate where the strips are located. The eigenvalue equations when the field is assumed uniform in the direction of the strips and in the case of narrow strips are then obtained as special cases of the analysis. Sample numerical results for both wide and narrow strips are also given.