Petri net is one of the effective modeling techniques which analyzes and designs concurrent and asynchronous systems. CPN is an extended Petri net which has color tokens. In this paper, we propose a new test case generation method using CPN. It transforms Estelle Specification into CPN, which is applicable to Design/CPN. It also generates UIO and subtour from OG and descriptor, which are resulted from Design/CPN. Using the proposed method, we can get more improved test coverage than existing methods. Therefore, more effective protocol conformance testing could be conducted. The test case generating method will be the basis of the automatic testing environmented.