Nonlinear distortion in injection-locked semiconductor lasers under direct modulation is numerically studied as a function of operational parameters. Significant reduction up to 25 dB in nonlinear distortion is observed compared with the free-running condition. The reduction in distortion is rather uniform over a broad range of modulation power at a fixed modulation frequency. The distortion reduction, however, varies dramatically with modulation frequency at a fixed modulation power due to the resonance effect. The reduction in distortion generally decreases with bias current level. The reduction in nonlinear distortion as a function of injection strength and detuning frequency is observed for the entire region of stable injection locking, suggesting that the injection locking scheme is a very effective method for nonlinear distortion reduction. However, the reduction for different nonlinear distortions is different and complicated, implying a trade-off for reduction in different nonlinear distortions to simultaneously happen.