An analysis is made for the initial current, the operation current limit, and the critical threshold current of the injection laser at continuous and pulse operation conditions. Both the heat generated at junction and bulk heat generation have been taken into account simultaneously, the later can be characterized with a normalized equivalent bulk heating resistance r. For the continuous operation case, it is shown that the bulk heat generation cannot often be neglected. In the analysis for pulse operation case, the effect of pulse duration and that of repetition frequency have been considered respectively. It is indicated that when both the high power and high repetition frequency operation are desirable, it will be optimum to choose the duty cycle approach to 0.03. In some applications where the repetition frequency is not an important factor, to make the repetition period just three times larger than the thermal relaxation time of lasers is suitable. As the repetition frequency increased sufficiently, the performance of lasers is determined by the duty cycle mainly. Examples of the analysis applied to some typical cases have been illustrated and the emission spectral band broadening originated from junction temperature variation during pulse operation is briefly discussed.