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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 fingering 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 efficiency 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.