Abstract: To evaluate principal generating indices of wind turbine generator (abbr. WTG) to quantify the ability of generating efficiency, it is necessary to select an index from these indices as the principal foundation for the monitoring of the service quality. Firstly, on the basis of historical SCADA data of a certain wind farm and by use of statistical analysis method, the statistical characteristic of principal generation indices of WTG such as availability, power generation ratio and load factor were analyzed and compared. Secondly, in the viewpoint of the source of indicator information the cause of making differences among indices was explained. Finally, combining with abnormal operating data, the sensitivity of generation performance indices to abnormal fluctuations in wind speed-power curve was compared. Comparison results show that although the power generation performance indices exert a certain consistency, however the power generation ratio is more representative for all power generation performance indices and can better reflect the power generation performance of WTG to incarnate the abnormal fluctuation of wind speed-power curve. Thus, as far as principal power generation performance indices of WTG, it is more suitable to use the power generation ratio for the principal index to be monitored.