Abstract: The packet loss rate is one of key indicators to evaluate the performance of network. A method for calculating packet loss rate in the measured data based on the intact TCP flows was presented. Based on the measured data, the packet loss rate of Jiangsu Province in CERNET network and different partitions outside was analysed. The algorithm gives the packet loss rate including first packet loss rate pfl, overall packet loss rate psl and retransmission rate prp. Through statistical analysis, pfl is smaller than psl in the vast majority of cases, and prp is significantly higher than pfl and psl, which proves the impact of packet loss on the performance of network, and retransmission rate is obviously higher than packet loss rate.