Although the effect of viscosity on oil recovery is well established, no systematic investigation has been implemented regarding the effect of oil viscosity changes on performance of carbonated water injection (CWI). In this study, the performance of CWI with different oil viscosities and rock wettability was investigated through two main series of high-pressure core flooding experiments on both clean and aged sand. The results obtained demonstrate that the capability of carbonated water to enhance oil recovery for both secondary and tertiary flooding is significantly greater versus that for water flooding. The creation of a low resistance flow channel and low oil recovery in water flooding is compensated for by CO2 diffusion and subsequent viscosity reduction and oil swelling in heavy and light oils. The results of the aged sand experiments showed that changing the wettability towards mixed wet caused ultimate reduction in oil recovery in each experiment, compared to the similar one in clean sand. However, it was observed that the amount of oil recovered after breakthrough compared to that in clean sand was increased, which showed the capacity of CW for enhanced oil recovery through alteration of wettability.