The distribution of shearing stresses at various parts of a valve spring subjected to compression was examined by means of small mirrors attached to the spring wire. The experimental results at each part of the spring wire agree fairly well to the value obtained from the formula, F=16WR/πrd3 where, F denotes the shearing stress in the spring wire, W the compressive load applied to the spring, R the radius of the spring, d the diameter of the spring wire, provided that the srping is subjected to a statical load. Under repeated compression, such as in the valve mechanism of high speed internal combustion engines, it was found that the spring coils vibrate at its natural frequency; it begins to “surge”. This is due to the resonance between the natural frequency of the spring and the frequency of the forced vibration as related to the revolution of the camshaft. How surge affects spring stresses was experimentally studied by means of small mirrors explained above; the total range of repeated stress in the region of about 2 coils from the end of the spring is especially high, amounting to about S times the calculated value from the formula. This is accordant with the fact that fatigue failures actually take place at this portion. Surge is a prolific source of valve spring failure due to the stress conditions and the rapidity of the stress cycle.