We will introduce some probabilistic methods to study heat kernel estimates for Sch\"odinger operators $L=-\Delta+V$, based on which the interaction between the behaviors of Brownian motions $\{B_t\}_{t\ge 0}$ and the potential $V$ will be applied. Our results include the case that $V$ is unbounded or $V$ is decaying to $0$ at infinity. Moreover, two-sided Green's function estimates associated with $L=-\Delta+V$ are also obtained.