A novel MAC protocol called adaptive beamforming time division multiple access (ABF-TDMA) for mobile ad hoc networks with smart antennas is proposed and analyzed ABF-TDMA is based on TDMA and smart antenna and takes the advantage of collision-free reservation. After successful reservation and training, data packet can be transmitted from source node to destination node with adaptive beamform. We analyze the throughput and delay in ABF-TDMA for the case of a fully connected network with variable-length packets and different number of nodes. The simulation results show that ABF-TDMA can achieve high throughput and keep low delay in the traffic-load ranges of interest, especially when the average packet length and the number of nodes are large.