A new method was proposed to analyze a basic rest-activity cycle (BRAC), termed BRAC-A. This method calculates a significant cycle length by chi-squared method and the starting points of a resting phase and an active one by analysis of the array consisting of the segmented time series in the same length. Using 20 3-day computer-simulated time series of standard deviation in fetal heart rate fluctuation with known periodicity of the BRAC, we demonstrated that the BRAC-A was a useful tool to reveal the characteristics of the BRAC.