Paralleled two-level converters are widely used in low-voltage permanent magnet direct-drive wind power generation systems， while three-level converters are more suitable in medium-voltage systems. The output performances and power losses are compared between two-level and three-level converters in permanent magnet direct-drive wind power generation systems. Loss models of power device and filtering inductance are established for evaluating the power losses of the converters. Based on the operating principle of two-level and three-level converters， a practical loss calculation method is derived. Simulation and experiment results show that， comparing with two-level converter， the three-level one features lower grid-side current harmonic components and less power losses.