This paper explores Input-Output Conformance (IOCO) test generation with Colored Petri Nets (CPN). A test generation oriented CPN model and CPN based IOCO relation is proposed. Feasible test cases are generated by model simulation with the proof of its soundness. The method integrates the merits the IOCO testing theory and the CPN modeling synergistically, and is applied as a nontrivial and competent test case generation approach for practical testing projects. The effectiveness of this test generation approach is demonstrated by a concrete software system. Since the model simulation based test generation process is irrespective with the model size, the effectiveness of the method is enhanced with scalability. © 2014 NSP Natural Sciences Publishing Cor.