The estimating system for the frequency stability was manufactured. The measurement, the data aquisition and the estimation of the stability, etc. are all controlled by a microcomputer PC 8001 mkII. Allan-variance and power spectral density was calculated as the measure of the frequency stability in the time-domain and the frequency-domain respectively. It was found that the tested quartz crystal oscillator had the stability of 1×10^<-11> at 1 sec averaging time and its power spectral density was designated by the sum of the terms of 6×10^<-25> f^<-2> (random walk noise) and 1.5×10^<-22> f^0 (white noise).