The present paper include three aspects of question. (1) how to determine the value of Maximum Lab M in theory? (2) what is the re:itionship hecween the length of sample series N and the period T which is contained in this sample series? (3) how much is reliability of the period T which is disclosed by spectra analysis? The value of Maxmum Lag M is expected to be high if spectra unbiased estimation is con-sidered. At the same time, however, this value is expected limited under the consideration of re-duction variance of spectra estimation. It is advisable then that the value of Maximum Lag M be selected as 1/3 to 1/10 of the length of sample series N in both theory and practical calcula-tions.