This paper presents an UAV (Unmanned Aerial Vehicle) system that can find the locations of the wireless devices such as smartphones and tablets in WiSAR (wildness search and rescue). By locating the wireless device, we can find the owner of it that get lost or get hurt in the wildness. The system contains a UAV armed with a wireless sniffer, a flight controller and data processing center. When the locating starts, the UAV will fly according to a planed route while the WiFi sniffer capturing the wireless signal from wireless devices. When the UAV discover the wireless device, a proposed algorithm will be executed to get the wireless device's distance and direction. And it will be executed iteratively until the UAV gets near enough. In order to avoid the environmental impact, we use the Kalman filter to process the RSSI (Received Signal Strength Indication). The experiments have proved that the proposed method is a fast and precise way to localize the wireless device.