With development of multi-core system, parallel encoder with fast algorithm is widely used to improve video encoding speed. In this paper, a dynamic slice-resize algorithm is proposed to balance workload and increase encoding speed for slice-level parallel encoder. This algorithm retrieves encoding workload from previous encoded frames and dynamically changes the slice size for the following frames to balance the encoding workload among slices. By applying the proposed algorithm, an average of 21% encoding speed improvement can be achieved for parallel video encoder without loss in objective performance.