Coexistent heterogeneous wireless networks may interfere with each other and result in significant performance degradation when devices are collocated in the same environment. With the increasingly deployed Wireless Personal Area Network (WPAN) and Wireless Local Area Network (WLAN) devices, channel conflict has become very frequent and severe when one WPAN technology coexists with other WLAN technologies in the same interfering range. In this paper, we study the coexistence issue between IEEE 802.15.4 and IEEE 802.11b. We present analytical models on the non-conflicting channel allocation probabilities, focusing on the coexistence scenarios of IEEE 802.15.4 coexisting with IEEE 802.11b. The interference model of IEEE 802.15.4 wireless Personal area network (WPAN) affected by IEEE 802.11b wireless Local area network (WPAN) also presented. The packet error rate (PER) of the IEEE 802.15.4 under the interference of the IEEE 802.11b is analyzed, and is obtained by the bit error rate (BER) and the collision time. The BER is obtained from signal to noise and interference ratio. The safe distance ratio can be obtained from the PER. The analytic results are validated for various topologies using Qualnet 4.5 simulation. Further this paper investigates the PER of IEEE 802.15.4, with the consideration of the mobility models of the WLAN nodes.