During the migration to next generation network, an important trend is that IP-based network will become the main vehicle for carrying audio and video applications. VoIP is a representative of such applications. However, packet loss is a main factor that degrades QoS of VoIP, and bursty loss jeopardizes QoS of VoIP much more than scattered loss. Packet dispersion is a mechanism that disperses packets between parallel paths following some dispersion strategies. It transforms bursty packet loss to scattered packet loss so as to improve QoS of VoIP. Although there are researches focusing on the use of packet dispersion in VoIP transmitting, the theoretic foundation of packet dispersion, such as to which extent packet dispersion can improve QoS and what is the optimal dispersion strategy, is unknown. In this work, concentrating on these problems, the quantitative relation between QoS of VoIP and the packet dispersion strategy is formulated as constrained optimization problems under Bernoulli packet loss model. Based on the closed form solution of the optimization problem, an algorithm is presented to find the optimal packet dispersion strategy. The proposed optimal dispersion strategy is verified under Gilbert packet loss model with ns-2 simulation.