Institute of Electrical and Electronics Engineers (IEEE) 802.15.4 is a protocol operating in media access control and physical layer, and it is commonly used in low-power and low data-rate wireless communications. In this paper, we develop a stochastic model for unslotted operations of IEEE 802.15.4 networks. Signal fading is an important factor in mobile communications. According to the level of fading, signalto-noise ratio gets dynamically changing values. Given a pair of nodes communicating to each-other in a mobile channel, the model calculates frame sizes providing maximum data-rate of successful IEEE 802.15.4 transmissions. The experimental results in various environments are given in the paper to discuss the success of the model. In the consideration of real-time operations, the model is improved to be operated in a lowcomplexity hardware/software unit. According to experimental results presented in the paper, the proposed model seems to be suitable in mobile environments by achieving high data-transfer rate for unslotted IEEE 802.15.4 protocol. of retransmission processes. Y.D. Lin and et al. studied to improve the network efficiency of WLAN by aggregating multiple frames into a single transmission [2]. Other studies given in [3,4] aimed to adjust the frame size in IEEE 802.11 networks. However, these studies are specific to the IEEE 802.11 protocol operations and not suitable for IEEE 802.15.4 based networks. A simple way to find optimum frame size is to decrease the size for each transmission failure and to increase the size for each successful transmission. Although implementation of this approach is easy, it is important to define the strategy of that how to change the values of the size. Approaches like [5,6] use channel statistics to find optimum frame size. However, these approaches are not suitable in IEEE 802.15.4 protocol because physical and medium access control (MAC) layers specifications of IEEE 802.11 and IEEE 802.15.4 protocols have different structures. Studies in [7, 8] present analysis of packet error rate (PER) using Markov models for various signal-to-interference ratios. Although PER is formulated for different scenarios, no adaptation is made to IEEE 802.15.4 protocol. Leea et al. propose an analytical model and present its performance using slotted carrier sense multiple access with collision avoidance (CSMA/CA) for IEEE 802.15.4 networks with acknowledging scheme [9]. However, this study includes no information about unslotted version of IEEE 802.15.4 networks. The authors of [10, 11] developed a stochastic model for unslotted IEEE 802.15.4 MAC operations. Although they analyzed the effect of the parameters such as duty cycles, collision and traffic rate in detail, no study is conducted for the channels that are effected by noise signals. W. Du and et al. studied the performance of IEEE 802.15.4 protocol for both slotted and unslotted operations [12]. A comprehensive analysis about packet delivery rate, latency, and energy consumption is given on their paper. An analysis of the ef...