Anti-islanding (AI) function is a necessity for distributed generation systems. Current injection based AI methods have been widely used. However, too small current injection may fail to detect the islanding, while too large current injection will lead to the disturbance and poor power quality. On the other hand, d-axis or q-axis current injection can be used, and one may wonder which is preferred? Therefore, this paper presents a quantitative analysis of them from the performance comparative viewpoint. It is expected to provide a helpful guidance for designing the effective AI function for distributed generation systems.