An effective algorithm to reduce gray-level disturbance (GLD) in pulse number modulation is proposed. GLD occurs when moving image sequences are presented by plasma display panels (PDPs) or digital micromirror devices (DMDs), which use pulse number modulation to express gray levels. We first develop a systematic model for GLD, and then show that GLD can be eliminated if the light emission pattern of every gray level has the same shape. Based on the ideal condition, we design subfield and driving vectors. The lexicographically largest vector is employed as the subfield vector, since it can flexibly control the shapes of light emission patterns. Then, three methods are proposed to determine driving vectors: the zero-order, first-order, and tree methods. The zero-order method has the lowest implementation complexity, whereas the tree method reduces GLD most effectively. The first-order method offers a good tradeoff between implementation complexity and disturbance reduction capability. Simulation results demonstrate that these methods suppress GLD effectively and provide good moving image quality.