A method of estimating TCP flow-rates of sampled flows through packet sampling is described in this paper. We use sequence numbers of sampled packets, which make it possible to improve markedly the accuracy of estimating the flow rates. Using an analytical model, we investigate how to set parameters such as packet sampling probability used in this method of estimation. As a remarkable result, we show that the estimation accuracy improves as the sampling probability decreases. Using measured data, we also show that this method gives accurate estimations. We also show that this estimation method enables us to detect performance degradation at the TCP flow level.