The optical power emitting from the cavity facet of blue InGaN-based laser diodes (LDs) is measured to investigate the efficiency droop. The efficiency droop behavior of blue InGaN-based LDs near the threshold is confirmed in our experiments. From measurements of optical power at different wavelengths, it is analyzed that the droop behavior of LDs can be ascribed to the efficiency reduction of longer wavelengths. The efficiency of longer wavelengths is subject to the carrier occupation process in quantum levels. In addition, it is found that the droop behavior may be largely affected by the relatively large threshold current of InGaN-based LDs and the screening effect of polarization, and it can be suppressed by stimulated emission.