Recently, integration of Distributed Generation (DG) in distribution systems has increased to high penetration levels. The impact of DG units on the voltage stability margins has become significant. Optimization techniques are tools which can be used to locate and size the DG units in the system, so as to utilize these units optimally within certain limits and constraints. Thus, the impact of DG units on some issues, such as voltage stability and voltage profile, can be utilized positively. The ultimate goal of this paper is to propose a method of locating and sizing DG units so as to improve the voltage stability margin. The load and renewable DG generation probabilistic nature are considered in this study. The DG units' placement and sizing is formulated using mixed integer non-linear programming (MINLP), with an objective function of improving the stability margin; the constraints are the system voltage limits, feeders' capacity, and the DG penetration level.