An experimental investigation was conducted to study the effect of applied stress on the increment of crack opening displacement (COD) under constant load condition. It has been shown in several investigations that the time-dependent increment of COD is observed at constant load after monotonic loading, when COD at the beginning of load holding is greater than δ _i , which is a critical value for fibrous crack initiation. In this paper, the effect of applied stress on the increment of COD at constant load was investigated on large sized center cracked specimens as well as on small sized ones. As a result, it was found that the time-dependent increment of COD is affected strongly by the applied stress level rather than the COD level, and that COD increases sharply at constant load beyond general yielding. The mechanism of time-dependent increment of COD under constant load condition was also discussed.