Abstract Steam-Assisted Gravity drainage (SAGD) is a popular approach for oil sands recovery, in which a steam chamber is developed inside reservoir with heated up bitumen flowing downwards along the edge of the steam chamber to production well by gravity. To improve the production performance of SAGD, solvent co-injection with steam (called ES-SAGD) is also proposed. In both SAGD and ES-SAGD, the penetration of low viscosity gas phase into viscous bitumen induces fingering at the steam chamber edge, resulting in non-uniform steam chamber. In this study, for the first time, the phenomenon of fingering is re-examined under the condition of mobile initial-water in oil sands through a newly designed reservoir simulation model. The effect of Solution Gas to Oil Ratio (GOR) on fingering in SAGD is also studied. In addition, the impact of solvent-water-bitumen phase behavior, which is critical to the amount of gas phase, on fingering is examined in ES-SAGD. The results reveal that fingering takes place at the oil zone on the top of steam chamber in SAGD, due to the ex-solution of solution gas under elevated temperature. The addition of solvent in ES-SAGD enhances the fingers by reducing the partial pressure of the solution gas and increasing the amount of solution gas at the top of steam chamber. It is found that the flow of the initial-water in oil sands mitigates the degree of fingering and helps stabilize steam chamber edge. This re-examination of fingering in SAGD and ES-SAGD demonstrates that the mobility of the initial water in oil sands has to be taken in account in the analysis of steam chamber instability.