In the tip clearance flow, the dominant vortex is the tip leakage vortex (TLV), which has a significant impact on the hydraulic and cavitation performance of axial flow machineries. In order to reveal the effect of gap size on the TLV, the gap flows with two gap sizes, i.e., t=0.2 (2mm) and t=1.0 (10mm), are numerically investigated. A NACA0009 hydrofoil is selected to create the gap flow, with an incoming velocity of 10m/s and an angle of attack of 10°. The results show that the two flow cases have big differences in terms of the vortex feature and the leakage flow distribution. For the small gap, a type of jet-pattern flow appears in the gap. While for the large gap, a type of rolling-pattern flow passes over the gap. The vertical velocity gradient of the leakage flow has a decisive influence on the TLV trajectory. In addition, for the large gap, the axial velocity in the vortex center exceeds the velocity of the incoming flow. This jet-like state of axial velocity can maintain for a long distance, making the vortex more stable. However, the axial velocity in the case of t=0.2 cannot stay at a jet-like state and rapidly switches to a wake-like state.