This paper describes an agent based model of a startup business firm for analyzing the conflict between the organizational performance and its employee motivation. Startup business firms tend to change its management strategies with the growth of the firm in order to increase the productivity and business performance. However, those changes may cause negative impacts on the motivation or entrepreneurship of its employee, and they might weaken the vitality of the firm for sustainable growth. According to those considerations, we have conducted the agent based simulation and have gotten the following suggestions. 1) Building management structures increases organizational performance while decreasing employee motivation. 2) Keeping the initial informal management style by not building a management structure makes employee motivation increase, however, it makes organizational performance decline. 3) Informal networks among diversified employees can ease the negative impact of
building a management structure.