Computer-based simulation games have been shown to be instructional effective in terms of knowledge improvement, self-efficacy, and retention (Sitzmann, 2011). Most of the extant research has been conducted in educational settings using student samples. Can computer based simulation games enhance employee job performance in the corporate world has yet been investigated. Using a field experiment with random assignment to experimental conditions and pretest-posttest measurements, the current paper reported a field experiment at IBM in which SimPM, a newly developed computer-based Simulation game for Project Manager, was tested for its effectiveness in training junior project managers to develop their on-the-job project management skills. Results of the experiment showed that participation in SimPM resulted in significant knowledge gain and higher job performance, comparing to a no-training control group. Participants in the simulation games also reported high levels of motivation and satisfaction. Implications for future research and practices on using computer-based simulation games for corporate training were discussed.