Broadcasting is one of the essential communication models of MANETs. Many MANET multicast routing protocols rely heavily upon MAC layer's broadcast support. However, the broadcast mechanism of the standard IEEE 802.11 cannot provide reliable broadcasting service. In this paper, we improve the IEEE 802.11 broadcast mechanism's reliability by introducing the adaptive round-robin acknowledge and retransmit (ARAR) scheme. Different from the other mechanisms, ARAR is able to adapt to the traffic load by adjusting the length of a frame transmitted. For short frames, a simple and effective DATA/BrACK (broadcast acknowledgment) scenario is applied; for long frames, RTS/CTS/DATA/BrACK exchange is utilized. Receiving the BrACK, the DATA sender calculates and records the indices of the frames that need retransmission and performs the retransmission when it is possible. The proposed scheme provides a highly reliable broadcast service to the routing layer, and the performance of multicast routing protocols can be improved significantly. This is shown through simulations, in which we have compared ARAR to the standard IEEE 802.11 approach using ODMRP [Y. Yi et al., Nov. 2002]. Our future work attempts to compare ARAR with some existing works.