The limited write endurance of phase change random access memory (PRAM) is one of the major obstacles for PRAM-based main memory. Wear leveling techniques were proposed to extend its lifetime by balancing writes traffic. Another important concern that need to be considered is endurance variation in PRAM chips. When different PRAM cells have distinct endurance, balanced writes will result in lifetime degradation due to the weakest cells. Instead of balancing writes traffic, in this paper we propose wear rate leveling (WRL), a variant of wear leveling, to balance wear rates (i.e., writes traffic/edudrance) of cells across the PRAM chip. After investigating writing behavior of applications and endurance variation, we propose an architecture-level WRL mechanism. Moreover, there is an important tradeoff between endurance improvement and swapping data volume. To co-optimize endurance and swapping, a novel algorithm, Max Hyper-weight Rematching, is proposed to maximize PRAM lifetime and minimize performance degradation. Experimental results show 19x endurance improvement to prior Wear Leveling.