The paper presents the effects of the leakage flow from the inter-platform gap on the;migration of hot streak in a first stage turbine by three-dimensional unsteady Reynolds-Averaged Navier-Stokes simulations. Five circumferential positions of the hot streak are considered. The comparisons between the results with/without slot leakage show significant differences. The leakage changes the hot streak in the vane passage significantly and it protects the vane suction surface trailing from the hot gas. The leakage also changes the secondary flow and results in forming a new couple of vortices in the vane passage. In general, rotor passage, the hot gas usually gathers in the rotor hub and the pressure surface. In the present study, the leakage coolant from upstream slot is entrained to the unsteady rotor secondary flows and transported toward the rotor hub and pressure surface effectively. The cooling effect is related to the relative circumferential positions between the hot streaks and slot. When the hot streak is positioned at the slot suction side, the time-averaged temperature reduction on the blade leading edge can be more than twice of that for the hot streak at the slot pressure side.