Round-Trip time (RTT) is an important metric for network measurement and an essential indicator for network performance monitoring. Traditional packet trace based RTT estimation usually depends on particular active or passive measurement platforms. This paper proposes a new RTT estimation method, which merely takes flow data from existed routers and hardly needs extra network measurement facility. Based on the analysis of transmission features of TCP bulk flow, RTT estimation models are established corresponding to the conditions where socket buffer size and bandwidth delay product (BDP) are relatively small, large and approximate. Experiments show RTT estimation can be well accomplished through those models. Moreover, considering only duration and total packet number of a TCP bulk flow are involved in estimation, this method is also adoptable to situation with sampling flow data as input, and thus is effective in monitoring and managing the large-scale backbone network performance.