Abstract: A fatigue test for micro-crack development was performed with a microscope and a large fatigue testing machine,combined with macro-analysis and microscopic examination.The crack initiation and propagation of small holes and the size effects of small defects on low cyclic fatigue of steel bar were investigated.The specimens chosen was 16MnR steel,normally for fabricating pressure vessels in China,with drilled holes of different sizes from 40 to 200 in diameter.It has been observed that one type of crack initiation mechanism at the edge of surface was caused by slip bands in which the shear stress dominates.The size of small defect and stress level affected the fatigue life apparently,but the effect weakened with increasing stress level.Micro defect sizes had significant effect on the fatigue life under low stress,but,there was no such significant effect on the fatigue life when the stress was higher than the yield stress.There was a critical value(dt) of the size of small defects beyond which (d>dt ),the fatigue life will dropped greatly.