In this paper, we describe scheduling improvement technique under timing constraints and resource constraints in high level synthesis. Scheduling is the process of assigning operations to control steps. For optimum scheduling, integer programming method can be used. However, only for small sized problems. List scheduling method is frequently used since it is efficient. The results of list scheduling may not be optimum. We developed a refinement technique to improve the results of list scheduling. For several examples, we could obtain the best results reported so far.