The reduced peak efficiency and the efficiency droop afterward, i.e. the degraded efficiency, of blue InGaN lightemitting diodes (LEDs) is investigated numerically. It is depicted that the joint effects of multiple factors, including the influences of polarization-induced electric field, the phenomenon of current crowding, and the Auger and ShockleyRead-Hall (SRH) recombinations, are responsible for the degraded efficiency. Among them, the severe SRH recombination due to the poor crystalline quality is the main cause of reduced peak efficiency, while the serious Auger recombination resulted from high Auger recombination coefficient and non-uniform carrier distribution of the active region is the major factor contributing to efficiency droop. It is shown that the strong built-in polarization field and the crowded current flow will result in the nonuniform carrier distribution, and thus enlarge the Auger losses and the efficiency droop.