Recent advances in Wireless Sensor Network (WSN) have facilitated the realization of pervasive health monitoring for both homecare and hospital environments. IEEE 802.15.4/ZigBee sensor network support small power consumption and node expansion compared to other network standards for WSN. Body sensor networks (BSN) require a large number of sensors for sensing the medical information from human body, and low power consumption for monitoring a patient's status for long time. Unfortunately, ZigBee has limited bandwidth and hard to support real time data transmission because of the adoption of CSMA-CA as its medium access control (MAC) protocol. Depending on the varying traffic loads, there are different back-off times for packet transmission. This affects the data loss rate and latency for data packets generated by a network node. Proper configuration is important in the successful operation of a ZigBee network. This paper analyzes the effect of different back-off parameters on the performance of beaconless operation of ZegBee MAC protocol.