Upper bound limit analysis method is applied to determine the required reinforcement for three-dimensional stability of the Slopes under seismic conditions. Horizontal blocks are used for determining the internal stability in three dimensional conditions. Seismic stability is studied by adopting a pseudo-static approach, considering only the horizontal acceleration. Reinforced soil has been considered to be a cohesionless material in the analysis. The failure mechanism is considered to be transitional and consists of several horizontal hexagonal blocks and a pentagonal one at the base. Velocity discontinuities between blocks are considered to be horizontal and each block consists of one reinforcement layer. Results are presented in graphical and tabular form to illustrate the effects of variation of different parameters such as seismic acceleration, three-dimensional geometry of the slope and soil friction angle on required reinforcement length and strength. By increasing of seismic acceleration, the stability of the reinforced soil slope decreases significantly, and thus greater strength and length of the reinforcement are required to maintain stability of the slope. On the other hand, three-dimensional modeling of reinforced slope results lower values of the required reinforcement. Comparisons of the present results with available pseudo-static results are shown, and discussed.