In wireless sensor networks, it is very important to obtain an accurate position of each sensor node. In this paper, we consider the potential of Ultra Wideband (UWB) signals for positioning due to its special characteristics. Accurate angle of arrival (AOA) can be obtained using UWB antenna arrays. We propose a new cooperative positioning method based on the AOA's. The method takes advantages of the pair-wise AOA's information among all the sensor nodes while existing positioning method only utilizes the AOA's from the anchor nodes. Then, the Cramer-Rao Lower Bound (CRLB) for Cooperative AOA localization is analyzed. Through the simulation experiments, we show that our proposed method could achieve higher positioning accuracy than existing non-cooperative positioning methods and its accuracy increases with the number of blind nodes.