Conformance testing is based on a test suite. Standardization committees release standard test suites, which consist of hundreds of test cases. The main problem of conformance testing is that we do not have enough time to execute them all. Therefore, test selection is required to maximize the test coverage. In our earlier papers 6,7 we outlined a new method of selecting an optimal test suite which can detect the errors with better probability and reduce the time of testing. In this paper we will expound the mathematical optimization method for test suite optimization based on cost and test coverage, and we will apply this method to an ISDN protocol.