This paper explores the optimal privatization of public firms in a mixed oligopolistic market under the framework of cost heterogeneity and nonlinear demand curve. It shows that the optimal degree of privatization is positively (negatively) correlated or irrelevant with the variance of marginal costs of domestic private firms as long as the inverse demand function is concave (convex) or linear. By contrast, the variance of marginal costs of foreign private firms will not affect home country's optimal policies of privatization.