IOSR Journal of Applied Physics, Vol.3, Issue 3, pp.30-40, ISSN No: 2278-4861. 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.