In the recent draft of the 802.1 le MAC , the Hybrid Coordinator (HC) gives transmission opportunity (TXOP) for a station to transmit burst frames in TXOP for improving throughput efficiency. In this paper. we express the expected throughput for the Block Ack policy in Polled TXOP as a closed form function of the number of burst MSDUs, the number of fragmented MPDUs, the data payload length, the frame retry count. the wireless channel condition, and the selected PHY mode. Based on our simulation study and analysis, we show that the TXOP and proper Ack policy can enhance system performance, and that there are the Block Ack efficient block size and frame length to transmit the burst frames for each PHY mode. Moreover, we also show that when the Block Ack mechanism is combined with link adaptation. it has better throughput performance.