Synergistic effect and “free-rider” effect in teamwork were analyzed. The principal-agent models for employee's independent work and teamwork were established.  It is concluded by comparing the principal's benefits from the different working that whether or not teamwork is superior to individual work lies on synergistic coefficient, variance of exogenous random variable, the team number's risk-averse parameter and the cost coefficient of effort.  When work properties assort with personnel character, accordingly the four factors satisfies definite conditions, it is beneficial to form personnel into a team; otherwise it is the optimal choice to let them work independently.