A heterogeneous network (HetNet) with macro base stations (BSs) together with low power small cells, such as pico BSs and femto BSs, is a promising solution to enhance network capacity. In HetNet, the transmission range of pico BSs are limited due to the large transmission power difference between macro and pico BSs, and thus pico BSs are typically underutilized. Cell Range Expansion (CRE) is a way to increase the opportunity of the user equipment (UE) associations to pico BSs so that more traffic from the macro cell can be offloaded to the pico cells. However, the expansion region of pico UEs with unbalance downlink and uplink signal qualities suffer from bad downlink quality, and the macro UEs near the expansion region may also experience low uplink rates. Therefore, we propose a new scheme with rate-based CRE offsets in HetNet so that UEs can decide to their associations based on their traffic demands. We formulate this problem as a mixed integer programming problem and prove that it is NP-hard. We then propose a polynomial time heuristic algorithm to solve this problem. The simulation results show that our scheme indeed achieves good traffic offloading gain for HetNet.