In this talk, we discuss the sharp two-sided estimates for the heat kernel of Dirichlet fractional Laplacian in open sets. This heat kernel is also the transition density of a rotationally symmetric -stable process killed upon leaving an open set. Our results are the first sharp two-sided estimates for the Dirichlet heat kernel of a non-local operator on open sets. This is a joint work with Zhen-Qing Chen and Renming Song.