The IEEE 802.15.4 protocol proposes a flexible communication solution for Low-Rate Wireless Personal Area Networks including sensor networks. It presents the advantage to fit different requirements of potential applications by adequately setting its parameters. When enabling its beacon mode, which uses the slotted CSMA/CA, the protocol makes possible real-time guarantees by using its Guaranteed Time Slot (GTS) mechanism and it provides reliability of the network. This paper analyzes the performance of the load in beacon enabled and non beacon enabled mode with variable node size. Variation in load metric is analyzed in star network by enabling and disabling Acknowledgement (ACK).The status of ACK also plays a critical role in analyzing load metrics.