Heavy oil and bitumen are expected to become increasingly important sources of fuel in the coming decades. Steam assisted gravity drainage (SAGD) is a commercially viable and widely used recovery technique for heavy oil and bitumen. However, it remains an expensive technique and requires large energy input in the form of steam. Energy intensity of SAGD, as well as environmental concerns such as fresh water usage and CO(2) emission, make it imperative to find new oil extraction technologies. Coinjecting a hydrocarbon additive with steam offers the potential of higher oil rates and recoveries with lower energy and water consumption. A reservoir simulation study using a 20 x 12 x 15 3D Cartesian model and Athabasca fluid and reservoir properties was conducted to evaluate this process. The role of hydrocarbon additive in the steam chamber and its effect on the performance of SAGD was investigated. Simulation results revealed the parameters that will have the greatest impact on the process performance and indicated the effectiveness of each hydrocarbon additive in improving the performance of SAGD. The results also showed that selecting the most suitable hydrocarbon additive depends on the operating conditions as well as the original reservoir fluid composition.