Miscible gas injection has been identified to be the most amenable enhanced oil recovery (EOR) process for Malaysian oil fields. Due to economic and availability reason, carbon dioxide (CO2) is the most potential source of gas for this process. In order to predict the performance of these fields undergoing miscible flooding using a reservoir simulation, there is a need to identify parameters that have significant effect on the results. Sensitivity study is usually conducted to evaluate the effect of varying parameters on the reservoir performance. This paper presents the results of the study using numerical simulation, with the emphasis on fluid properties since these data are usually obtained from laboratory measurements or correlations, which generally subjected to great uncertainties. Therefore, it is the main objective of this work to determine the fluid properties that have major impact on the oil recovery from miscible CO2 injection. Using an extended black oil simulator, the effect of varying oil and injected gas density, viscosity and formation volume factor, on the flood performance is assessed. The effect is quantified using an index called normalized root-mean-square deviation and a fractional change index. From the results of the simulation, the model is found to be most sensitive to fluid formation volume factors, followed by their densities and least sensitive to the fluid's viscosities. Thus, it is recommended that these data must be properly selected and analyzed beforehand in order to minimize their uncertainty prior to any simulation works.