[show abstract][hide abstract] ABSTRACT: Quality of service requirements for IP networks are becoming higher. Accordingly, managing, measuring, and monitoring IP networks are also becoming more important. In particular, for real-time communication, the quality of packet transmission (packet loss, delay, and delay variation) needs to be carefully maintained. In managing these performance parameters, active measurements have been often used, and measured samples are periodically evaluated statistically. Although performance measurements are standardized by several organizations, such as ITU-T and IETF, the length of the measurement intervals have not been mentioned. In this paper, in order to consider its appropriate length, we conduct periodogram analysis of packet transmission delay and investigate its stochastic characteristics in the frequency domain. After considering long-range dependency and randomness arising in packet transmission delay, we develop a method to determine the optimal measurement period for statistification. We applied our method to measured samples in an actual network and verified the effectiveness of our method.