This paper presents a smart ZigBee indoor security system based on the small-scale fading effect of RF signals. The received signal strength in a wireless network stays stable with low variances in a closed indoor environment. The movement of objects will induce a small-scale fading effect, resulting in dramatic changes of the signal strength. The main idea is to deploy a ZigBee network in the detection area and monitor the received signal strength of the signals in the network to detect environment changes. By analyzing the small-scale fading effect, we propose the simple rolling-window algorithm. Experiment results demonstrate the ability of the system to detect moving objects accurately.