Optimum solvent composition is a vital decision in ensuring the economic viability of the VAPEX process. The ideal optimum is dictated by reservoir characteristics such as temperature, pressure and bitumen properties. Condensation can be prevented through the addition of a non-condensable gas and if the partial pressure of the solvent mixture remains lower than the vapor pressure at reservoir temperature. In this work, different solvent mixtures were examined to investigate the performance of VAPEX process in heavy oil recovery in an Iranian reservoir. The optimum solvent composition was found through the comprehensive simulation work. Effects of injection rate and diffusion coefficient were tested. Optimum solvent composition for all cases was found as mixture of C1/C3 with 55/45 molar ratio. Results indicate that the recovery factor, the oil production rate and the cumulative oil production will be higher for solvent mixtures with diffusivity coefficient compare to solvent mixtures without diffusivity coefficient as well for optimum solvent. Effect of diffusivity on process is acknowledged well by peclet dimensionless number and this shows that at low injection rate, effect of diffusivity is more than displacement.