Gas injection plays an important role in bubble formation in microfluidics. The effects of the gas feed on the formation of N-2 bubbles in a microfluidic T-junction is highlighted under constant-pressure injection and constant-volumetric-flow-rate injection for gas. The effects of gas feed on characteristic parameters for bubble formation are studied, such as length l(B), velocity u(g), and frequency f of bubbles. The results indicate that the characteristic parameters increase with inlet gas-liquid flow rate ratio or pressure ratio. However, the parameters are more sensitive to the variation of inlet pressure ratio. The difference lies in the feedback effect of two-phase flow in downstream microchannels, which can be included in the relationship between the inlet pressure ratio and flow rate ratio. Finally, guidance for selecting gas feed method for gas-liquid two-phase flow in a microchannel is presented.