The relation between radiation quantum efficiency and current density of blue light LED&LD was studied. LED radiation quantum efficiency is very high at low current density, and reaches the peak value when the current density is 24.4A/cm2. But the quantum efficiency declines sharply as the current density increases. LD radiation quantum efficiency is zero when the current density is lower than the threshold, but increases rapidly after the current density reaches the threshold, and surpasses that of LED when the current density reaches 4.85×103A/cm2. Analysis shows that the huge drop of LED efficiency is caused by its own light-emitting mechanism, which restricts the promotion of radiation rate. The stimulated radiation emitting mechanism of LD makes up for the deficiency of LED, and the rate of no-radiative recombination has a relatively low impact on the radiation quantum efficiency. The results show that LD has characteristics of high radiation quantum efficiency under high current density.