AC line reactors and dc link reactors are widely used in variable frequency drives(VFDs) to improve the drive performance such as reducing input current harmonics, elevating input power factor, and protecting the drives from surges, etc. The effectiveness of both types of reactors in reducing input harmonics is affected by the loading of the drives and the system source impedance. In this paper, a simulation is performed to investigate of such factors. The rules to evaluate the needs and effectiveness using ac line or dc link reactors are proposed for practical appications.