Broadcasting is a fundamental and essential data dissemination mechanism in Mobile Ad hoc NETworks (MANETs). This paper presents an efficient probability-based broadcast scheme. In the scheme, to accelerate the packets propagation, the nodes with higher velocity are selected with higher probability to rebroadcast the received packet. Moreover, to reduce transmission redundancy and collisions without decreasing packet delivery ratio, the rebroadcast probability at each node is dynamically and adaptively adjusted according to neighbor density. The velocity and neighbor density-based rebroadcast mechanism keeps the scheme insensitive to the ever-changing network environment, even to the high mobility environment. The theoretical analysis and the simulations show that the proposed scheme outperforms the existing broadcast schemes in packet delivery ratio and normalized broadcast overhead, especially in average end-to-end delay.