A Mobile Ad hoc Network (MANET) is a collection of terminals equipped with one or several wireless network interfaces. In this paper, we suggest an improvement of the Transmission Control Protocol (TCP), called Improvement of the Acknowledgement mechanism of TCP (IA-TCP), to better performance in MANET. IA-TCP delays the acknowledgements packets of TCP according to the number of nodes, their mobility and the communication distance between these nodes. We limited our studies on very important parameters in such networks which are throughput and end-to-end delay because they have great effects on the TCP performance in MANET. The results are satisfactory and showed that IA-TCP can outperform not only TCP standard, but also similar techniques that have been proposed in the literature like LDA and WCCP.