Abstract: The range query of moving objects’ location is very important in many mobile applications, especially in analyzing, decision making, predicting, etc. Real-time locating system (RTLS) is a mobile system using RFID technology with the feature of skew object density. There are always storage wastes or performance decline while using existing indices in real-time locating system because of the skew object density. In this paper, a novel index mechanism called RPI (region partition index) is proposed to answer the range queries in RTLS. It firstly divides the region of the RTLS into sub regions according to the object density, and then indexes the division regions with R-tree. The object locations in these division regions are indexed by grid. Furthermore, this index is optimized to be cache conscious. In the optimized index structure, the object locations in a grid cell are stored in a list of arrays. The size of each array is determined by the size of the CPU cache line. Experimental results show that the new index has better search performance than R-tree and grid, and still keeps quite prominent update performance while object density is skew. The optimized index also brings strong performance improvement because it sharply reduces the cache miss rate in range queries.