Abstract: The paper discusses the impact of cost of signal on the implementability of social choice rule. The result of the paper shows that,in environment with moral constraint,monotonicity is nolonger a necessary condition for Nash implementation.In environment with moral constraint,when the number of agents is no less than 3,any social choice rule that satisfies unanimity is Nashim plementable,and when the number of agents is 2,any social choice rule that satisfies unanimity and intersection is Nashimplem entable.According to the results of this paper,the number of social choice rules which are implementable may be larger than we have thought.In environment with moral constraint,some social choice rules which do not satisfy monot onicity but unanimity,such as Pareto rule and Borda rule,are also Nash implementable.

Key words: social choice rule, Nash implementation, cost of signal