Abstract: The analysis of tooth root stresses is a principal subject in gear design. With operation of gears, the distribution feature of stresses is an important index in evaluating the performance of gearing. This paper introduces an approach to calculation of tooth rigidity, load distribution and tooth root stresses for spur gear. The experimental analysis for both modified and unmodified teeth has been made with comparision. The experiment proved that the dynamic characteristic of tooth root stresses can be much improved by means of tooth profile modification for heavy .load gearing.