The threshold stress intensity factor (Delta K-th) of small crack is affected by various factors, especially by material hardness, stress ratio and crack size. Test results showed that harder material had higher Delta K-th and smaller crack had lower Delta K-th. The crack closure measurement on a very small crack was done to make clear the root cause of those effects. Most of those effects could be explained by the peculiar behavior of crack closure for crack deeper than 100 mu m. However, everything could not be understood only by the crack closure behaviour. In addition to the difference in crack closure, (Delta K-eff)(th) itself was also dependent on crack size when the crack depth was shallower than 100 mu m. Another remarkable phenomenon was experienced in this study. Unusual decrease in Delta K-th was found in some case. The large decrease of Delta K-th occurred under the conjunction of three factors, that is, extremely high stress ratio (R) higher than 0.8, small crack and hard material. This kind of large reduction in Delta K-th in high R region is of much importance in turbo machinery that operates under high mean stress with small vibratory stress.