In the present paper, authors have performed an experimental study on fatigue crack propagation at threshold level to clear the effects of stress ratio and stress history upon the threshold stress-intensity factor ( K -threshold). Fatigue tests were carried out under load controlled three-point bending condition using side notch type specimens. The K -threshold values under the different stress ratio conditions are obtained by load decreasing test method which can eliminate the influences of loading history. The results show that the K -threshold range decreases with the increasing of stress ratio. An empirical equation which represents the relation between K -threshold range and stress ratio is proposed based on the comparison among present results and other researchers' data. The K -threshold values after crack propagation at constant stress range are obtained by load increasing test method under various stress ratio conditions. It is found that the K threshold value after stress history increases with the increasing of stress range at stress history process. And this tendency is well characterized by using maximum stress-intensity factor at stress history process and at threshold. A simplified equation for the prediction of K -threshold value after given stress history of constant stress range is derived from the present study. Estimated K -threshold values by proposed equation show fairly good agreements not only with present results but also with other researchers' data.