We have proposed a mobile multimedia satellite communication system designed to realize high-speed multimedia communications. This system uses a mobile network for the return link and the newly developed tracking antenna that receives high-speed satellite signals for the forward link. This paper describes IP network performance evaluation results of the proposed system in mobile environments. First, we measured round trip time and TCP throughput and confirmed the coincidence between experimental and theoretical TCP throughput that can be obtained from round trip time and TCP window size. Then, in order to make throughput recovered right after a shadowing environment caused by buildings, trees, etc. in which the tracking antenna can not receive satellite signals, the XTP (Xpress Transfer Protocol), which is designed to boost throughput for long latency networks, was incorporated into the proposed system and throughput tests were carried out. We confirmed that the throughput was improved and almost reached the network limit by the tests. We also clarified that the XTP is effective even in a low network quality that includes shadowing environments.