Compatibility verification for modified software is very important. Currently this is done by executing existing test cases to the modified software. However it is often difficult to perform enough verification because it all depends on the existing test cases. In this paper, a novel compatibility testing method for software logical behavior is proposed. The method does not require existing test cases. Instead, it generates test cases which cover all possible paths in both the existing software and the new software automatically by using Symbolic Execution, and executes them to achieve exhaustive compatibility verification.