This paper mainly studies the effect of design patterns on the Software maintainability. Design patterns describe solutions for common design problems and they were introduced to improve software quality and accelerate software development. However, there are some difficulties to choose an optimal pattern adapted to a certain application and problem. So until now the results on the effect of design patterns on software quality are controversial. In this context, we propose a tool for design pattern guided that retrieves the appropriate pattern with respect to software maintainability from a repository of patterns. It measures the maintainability of design pattern by some metrics and candidate the more maintainable pattern to the designer or developer. It provides a support for decision making during system design and refactoring. As the results, the decision of applying a certain design pattern is usually a trade-off since the effect of design pattern on software maintainability is influenced by some factors such as the pattern size and the prior expertise of the developer.