Managing the performance at the flow level through traffic measurement is crucial for effective network management. On the other hand, with the rapid rise in link speeds, collecting all packets has become difficult, so packet sampling has been attracting attention as a scalable means of measuring flow statistics. We have therefore established a method of detecting performance degradation at the TCP flow level from sampled now behaviors. The proposed method is based on the following two flow characteristics: (i) sampled flows tend to have high flow-rates and (ii) when a link becomes congested, the performance of high-rate flows becomes degraded first These characteristics indicate that sampled flows are sensitive to congestion, so we can detect performance degradation of flows that are sensitive to congestion by observing the rate of sampled flows. We also show the effectiveness of our method using measured data.