Abstract: To maintain a balanced injection and production rate in ultra - low permeability reservoirs is very difficult even when applying a very high pressure water injection, due to the very fine pore throats, therefore the feasibility of gas injection should be considered. Water - drive and nitrogen - drive experiments are performed to compare their displacement effects and study the displacing mechanism of ultra - low permeability reservoirs. In core with an ultra - low permeability of 1×10-3μm2 , the average recovery factor of gas drive is 27. 9% , 3. 8 percentage points higher than the 24. 2% of water drive, indicating that gas drive is advantageous to some extent.