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CO₂ flooding is considered not only one of the most effective enhanced oil recovery (EOR) methods, but also an important alternative for geological CO₂ storage. In this paper, the visualization of CO₂ flooding was studied using a 400 MHz NMR micro-imaging system. For gaseous CO₂ immiscible displacement, it was found that CO₂ channeling or fingering occurred due to the difference of fluid viscosity and density. Thus, the sweep efficiency was small and the final residual oil saturation was 53.1%. For supercritical CO₂ miscible displacement, the results showed that piston-like displacement occurred, viscous fi ngering and the gravity override caused by the low viscosity and density of the gas was effectively restrained, and the velocity of CO₂ front was uniform. The sweep effi ciency was so high that the final residual oil saturation was 33.9%, which indicated CO₂ miscible displacement could enhance oil recovery more than CO₂ immiscible displacement. In addition, the average velocity of CO₂ front was evaluated through analyzing the oil saturation profile. A special core analysis method has been applied to in-situ oil saturation data to directly evaluate the local Darcy phase velocities and capillary dispersion rate.