With the prevailing of mobile phones and online video contents, the demand for mobile online video is increasing. The desire, however, is held down by the high mobile traffic cost. To solve this problem, an off-the-shelf solution is WiFi-Direct (WFD), which is widely available on a majority of mobile devices. There is, however, no systematic study of WFD-based group data transfer and cooperative video streaming on real phones. Thus, we designed and implemented a WFD-based LTE cooperative video streaming system, in which the WFD GO (Group Owner) device takes the responsibility of peer information exchange, data relay, and LTE cooperative downloading scheduling. Based on the system, we evaluated the performance of WFD-based group data sharing, including Ping response delay, throughput, and power efficiency. Valuable findings were obtained. For instance, we discovered that when a WFD device connects to a traditional AP (Access Point), even if there is no data transmission to/from the AP, the Device-to-Device (D2D) throughput would decrease by at least 72%. Based on these findings, we provided recommendations for the design and deployment of WFD based D2D systems. We finally demonstrated the feasibility of WFD based LTE cooperative video streaming using our system. We showed that, using multiple realistic LTE networks, a 3-device cooperative system can provide smooth video streaming with bitrate more than 10Mbps.