Trailing-edge (TE) serration is a widely used instrument for noise control on wind turbine blades. Most of previous studies on TE serrations focused on noise reduction, but its aerodynamic effects on wind turbine performance have not received much attention yet. In this study, effects of serrations on the performance of a 2.3 MW wind turbine, including the power output and load characteristics, are examined using the airfoil analysis tool Rfoil and wind turbine analysis code Bladed. An optimal design method setting maximum power output as the design object is applied to determine the configurations of serrations regarding the airfoils corresponding to two sections in the last 20% span. Compared with the baseline airfoils, the serrated airfoils have higher lift-to-drag ratio at the angle of attack under the rated wind speed. For a full-scale wind turbine, the presence of serrations can slightly increase the static electrical power and the load at the blade root before the rated wind speed is achieved, and consequently increase the annual power output by less than 0.7%. However, under the turbulence wind conditions, the serrated blade has larger mean and maximum moment at the root and stronger load fluctuation, since the presence of serrations increases the angular moment of the airfoil. The analysis results suggest that using optimal designed TE serrations can achieve reduction of noise and increase of power output simultaneously, at the cost of increase of blade load.