TCP has a long slow start duration and low utilization in that duration, especially in long distance high speed networks. In this letter, we propose a new attainable TCP throughput measurement tool to minimize the effect of a transient state by detecting TCP states and excluding the transient state. The evaluations are conducted on both the testbed and the Internet, and the results verify that the estimated throughput by the proposed method is closer to the actual bandwidth than that of Iperf, while maintaining the same measurement time in long distance high speed networks.