Abstract: Tight oil is an important unconventional hydrocarbon resource. Its low recovery of primary production induces the necessities of enhanced oil recovery researches. Due to the complex pore structure of tight oil reservoir, the contribution of different pores is different. So it is necessary to study effective displacement in different pores and optimize development mode. In this study, nuclear magnetic resonance(NMR) and core flood tests were conducted to experimentally simulate different displacing methods(CO₂ flooding and waterflooding), and to evaluate effective displacement of different pores. The tests results indicated that effective displacement of different pores of CO₂ flooding and waterflooding increases with an increasing pressure drawdown. Most of the oil in large pores was displaced. As the pore radius turned small, displaced oil gradually decreased. In the pores with radius less than 0.01 μm, effective displacement of CO₂ flooding was better than that of waterflooding. In tight oil reservoir CO₂ was easier to enter the small pores to enhance oil recovery.