Abstract: An effective combinatorial test case generation algorithm called in parameter order_T-way (IPO_T) is proposed based on in parameter order (IPO) algorithm. In order to achieve the optimization of test suite, IPO_T algorithm determines the method of horizontal growth according to t-way(t≥2) combinations that are covered by the extended test cases, and revises extended test cases depending on the new covered t-way combinations. A test case generation tool is designed and implemented with this approach, and compared with some existing tools. Experimental results show that IPO_T outperforms them in terms of the number of generated test case within reasonable execution times.