Reducing operation and the maintenance costs of wind turbines has become a primary issue of wind farm owners and operators. Since the supervisory control and data acquisition (SCADA) system has been widely used in wind farms, it is costeffective to use SCADA data to realize condition monitoring. To this end, this paper proposes a method to calculate health index of wind turbines based on SCADA data. This method uses kernel density estimatison (KDE), then calculate the relative health index (RHI) of all wind turbines in a certain wind farm. To show the effectiveness of the method, we apply our method to real SCADA data of several wind farms. The result shows that RHI can reflect the health status of each wind turbine. Therefore, it is convenient to maintain the turbines with low RHI, thus to save maintenance costs.