The paper presents a test suite reduction approach based on probability models in regression testing. First, we model the process of software testing by using the joint probability of both test cases and program faults, and propose two probability models for the software's old and new versions, respectively. Then, a theorem combining set operations and probability models is given to implement our approach. Different from the traditional coverage-based test suite reduction methods, the reduced test suite constructed by our approach does not need to cover all test requirements, but also remains the same fault detection capability of the original test suite.The paper presents a test suite reduction approach based on probability models in regression testing. First, we model the process of software testing by using the joint probability of both test cases and program faults, and propose two probability models for the software's old and new versions, respectively. Then, a theorem combining set operations and probability models is given to implement our approach. Different from the traditional coverage-based test suite reduction methods, the reduced test suite constructed by our approach does not need to cover all test requirements, but also remains the same fault detection capability of the original test suite.