The stability o f natural convection in a dielectric fluid-saturated vertical porous layer in the presence o f a uniform horizontal AC electric field is investigated. The flow in the po rous medium is governed by Brinkman-Wooding-extended-Darcy equation with fluid vis cosity different from effective viscosity. The resulting generalized eigenvalue problem is solved numerically using the Chehyshev collocation method. The critical Grashof number G,, the critical wave number a,, and the critical wave speed cy are computed fo r a wide range ofPrandtl number Pr. Darcy number Da, the ratio o f effective viscosity to the fluid viscosity A, and AC electric Rayleigh number R,.a. Interestingly, the value o f Prandtl number at which the transition from stationary to traveling-wave mode takes place is found to he independent o f Rea. The interconnectedness o f the Darcy number and the Prandtl number on the nature o f modes o f instability is clearly delineated and found that increasing in Da and Rea is to destabilize the system. The ratio o f viscosities A shows sta bilizing effect on the system at the stationary mode, but to the contrary, it exhibits a dual behavior once the instability is via traveling-wave mode. Besides, the value o f Pr at which transition occurs from stationary to traveling-wave mode instability increases with decreasing A. The behavior o f secondary flows is discussed in detail fo r values o f physi cal parameters at which transition from stationary to traveling-wave mode takes place.