In this paper, an optimized Coding Tree Unit (CTU)-level rate control approach is proposed for low-delay high efficiency video coding (H.265/HEVC). Unlike the traditional explicitly estimated rate-distortion (R-D) model, the distributions of parameters of the estimated R-D model are considered. Accordingly, we formulate the CTU-level rate allocation for H.265/HEVC as the decision making problem, where the objective is to minimize the summation of CTUs' expected distortion under the given rate constraint. Moreover, a two-stage Bisection based method is proposed to solve the optimization problem, as the rate control algorithm for H.265/HEVC. The R-D performance improvements on Bjøntegaard delta bit rate (BD-BR) demonstrate the advances of the proposed method.