A method and system for improving the working set of a program image. The working set (WS) improvement system of the present invention employs a two-phase technique for improving the working set. In the first phase, the WS improvement system inputs the program image and outputs a program image with the locality of its references improved. In the second phase, the WS improvement system inputs the program image with its locality of references improved and outputs a program image with the placement of its basic blocks in relation to page boundaries improved so that the working set is reduced.