In the present study, an economic design of Anchored Geosynthetic(AG) System applied mainly to reinforce unstable soil slopes is investigated. For this purpose methods of stability analysis are developed to determine the optimum installation angle, required minimum length and maximum spacing of nails. Anchorage of nails within the soil mass is achieved by frictional resistance to pull out along the effective length of the nails. Cases of infinite slope and finite slope are dealt with individually. Silce methods of stability analysis developed in the present study are limit-equilibrium-based. For the case of finite slope Spencer method which considers interslice force is modified to evalyate the overall stability. In addition, the effects of various design parameters on requried length and spacing of nails corresponding to the optimum orientation of nails are analyzed. Based on the analysis, a simplified equation is given for the optimum nail orientation. Also the importance of optimum nail orientation is illustrated throughout design example, and the appropriateness of judgment criterion are examined.