This research aims to further understand the CO2 flow behavior during CO2 flooding. Here, we used NMR to monitor oil saturation during core flooding experiments and volume of fluid method to calculate CO2 displacement during simulation. The results indicate that during miscible flooding, piston displacement occurred in the early stages, followed by viscous fingering. Oil was first produced from macropores, followed by micropores and micropores. However, during immiscible flooding, viscous fingering occurred and gradually intensified until CO2 breakthrough. In these tests, oil was almost exclusively produced from macropores. The VOF results showed that viscous fingering with tip splitting is predominant in immiscible flooding. Piston displacement flow prevails near the core upstream and then evolves downstream into parallel viscous fingering in miscible flooding. Furthermore, increasing viscosity leads to a transition from formation of densely distributed, parallel, viscous fingers to growth of a few discrete, well-developed, dominant fingers. Increasing of injection rate brings the CO2 flow patterns closer to parallel dense viscous fingering, which improves sweep efficiency in midstream and downstream but slightly reduced sweep efficiency in upstream. On the other hand, a strong preferential flow channel forms when dominant viscous fingering occurs, which leads to a decrease of sweep efficiency. •Oil recovery and CO2 flow of CO2 miscible flooding and immiscible flooding were studied using VOF simulation method and NMR technology.•Increasing injection rate lead to rapid formation and merging of multiple viscous fingers, promoted the overall sweep efficiency of CO2, thereby improving oil recovery.•Increasing oil viscosity reduces the degree of miscibility, causing serious viscous fingering phenomenon.•Piston displacement near the upstream and fingering near the downstream in miscible flooding.