As pointed out in the “Discussion of Results”, the sub-surface stresses in a gear tooth decrease very rapidly with its depth. This result serves as a supporting evidence to the common practice of case-hardening the gear teeth. Previously this was done according to the mechanical properties of the gear materials, but without any analytical basis. Gear stress is a very complicated problem which includes in general the static, dynamic and fatigue stresses. Most of all, many uncertainties in service such as the impact load, acceleration load and work hardening are involved. These uncertainties make it difficult to get a general solution. The photoelastic method is an effective one for making a static study of gear stress, but it is understood that this method can not solve the whole problem, Since if the static stresses are considered further studies covering the notch sensitivity, friction effect, dynamic effect, and load distribution are still needed. / Master of Science