Simplified equations for peak position and for its standard deviation resulting from counting statistics are derived in the n points parabola method. These equations are applicable to an even as well as an odd n and can be rapidly calculated even with an inexpensive on-line computer such as a microcomputer. Thus a rapid and precise peak determination can be made by use of these equations with an inexpensive X-ray stress measurement system. Various peak positions calculated by the n points and the three points parabola methods were compared. Two heat-treated steels were used. The n points method, using all the data points that fall above approximately 85% of the maximum intensity, gave the most satisfactory result. On the other hand, the three points method produced a large standard deviation of the peak position unless the three points were taken symmetrically about the diffraction peak.