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Fatigue tests carried on three sets of samples having different mean grain sizes revealed that fatigue strength is a function of mean grain size of the rock. Samples having smaller grain size show higher value of fatigue strength. Graywacke samples from Flagstaff formation having mean grain sizes of 1.79 mm, 1.35 mm and 0.93 mm showed fatigue strengths of 87%, 88.25% and 89.1% respectively. Since the mean uniaxial compressive strength also varied with varying grain size, i. e. higher mean strength value for samples having finer grain size; the fatigue strength of a rock also shows a converse relation with mean uniaxial compressive strength.