In high pressure turbines, film cooling methods are used to reduce the metal temperatures of the blade. Understanding the mixing of the main flow and the coolant is key for the development of better cooling designs. It was found that surface film cooling effectiveness distribution were different with a laminar or a turbulent approaching main flow boundary layer, but the physical mechanism was not fully understood. In this paper, large eddy simulation (LES) method is used to investigate the effects of the approaching main flow boundary layer on the flow physics and cooling performance of inclined jets in cross flows. The different mixing processes of the coolant with different boundary layers status of approaching main flow are investigated with the instantaneous flow field provided by LES method. With a laminar approaching boundary layer, the horseshoe vortex develops upstream of the cooling hole. The coolant mixes with this horseshoe vortex and cools the area underneath the horseshoe vortex near the cooling hole. With a turbulent approaching boundary layer, the horseshoe vortex is not evident, and the distribution of film cooling effectiveness changes near the cooling hole. Although the distributions of time averaged film cooling effectiveness become similar for both approaching boundary layers on the area 5d downstream of the cooling hole, the current study found that the coolant mixing process are different. (C) 2016 Elsevier Ltd. All rights reserved.