In the IEEE 802.11 medium access control (MAC) layer, the reliability as well as the throughput is also important. The fragmentation and aggregation (F&A) algorithm to improve the throughput while ensuring the reliability in the environment with error uses the MAC service data unit (MSDU) fragmentation scheme of the upper layer and the MAC protocol data unit (MPDU) aggregation scheme of the MAC layer. The aggregation technique reduces the overhead of the frame format and improves the throughput. The fragmentation technique increases the transmission success probability and improves the reliability. When using F&A algorithm, the network throughput can be further improved. Although there were a lot of research on the F&A algorithm, these didn't cover the fragment size. Then it is left as user's choice. Furthermore, there is always a tradeoff between fragmentation and aggregation for wireless local area network (WLAN). We study this tradeoff and propose an optimal fragment size for aggregate-MPDU (A-MPDU), which can maximize the throughput by selecting the optimized fragment length based on different channel conditions.