In-Situ Combustion (ISC) is one of thermal heavy oil recovery methods in which the heat required to displace crude oil is generated by combustion of a small fraction of oil inside the reservoir. Because of presence of several processes such as combustion, phase change and reservoir fluids thermal expansion, in-situ combustion is regarded as a very complicated recovery method. In the present work, aiming acquiring a better understanding of ISC physics, the oil in place volume (expressing in terms of oil saturation) effects on performance of ISC is numerically investigated in 1D. In order to increase the model accuracy, a semianalytical model is used to account for heat loss to overburden and underburden. The numerical results show in reservoirs with high initial oil saturation, the mobilized oil is deposited in region near to production well during first days of ISC operation. Consequently, relative permeability of porous reservoir for gas phase considerably decreases. Moreover, combustion front propagation velocity reduces and the reservoir pressure significantly increases in the region upstream of the combustion front. As a result of the front velocity decrease, oil recovery rate decreases. Furthermore, if the pressure increasing is not considered in designing the air injection system, the air injection rate will be decreased and can lead to combustion front quenching. The results also show ignoring heat loss from the reservoir will lead to incorrect prediction of pore blockage.