Phase-change memory (PCM) is a promising candidate as a storage medium to resolve the performance gap between main memory and storage in battery-powered mobile computing systems. However, it is more expensive than flash memory, and thus introduces a more serious storage capacity issue in low-cost solutions. This issue is further exacerbated by the fact that existing file systems are usually designed to trade space utilization for performance over block-oriented storage devices. In this work, we propose a multi-grained block management scheme to optimize the space utilization of file systems on PCM-based storage systems. By utilizing the byte-addressability and fast read/write feature of PCM, a methodology is proposed to dynamically allocate multiple sizes of blocks to fit the size of each file, so as to resolve the space fragmentation issue with minimized space and management overheads. A series of experiments was conducted to evaluate the efficacy of the proposed scheme, and the results show that the proposed scheme could significantly improve the space utilization of file systems.