Abstract— This paper evaluates the performance of IEEE 802.15.4 standard Wireless Sensor Network (WSN) in star topology for large scale applications. The OPNET simulator (version 14.5) is used to simulate and analyze the performance of the network in terms of the number of nodes, packet size and Packet Interval Time (PIT). Starting with a packet size of 1408 bit and PIT of 1s, network performance is evaluated for different number of nodes; and a comparison among different network performance parameters (number of nodes, packet size and PIT) is made in order to choose the best optimized network performances concerning throughput, end to end delay, and packet drop. Simulation results show that the best performance is obtained at 230 nodes number with a packet size of 2048 bits and PIT equals 2s.