Power fluctuations in the main mode of a nearly single-longitudinal-mode AlGaAs laser are investigated. Average CW mode powers were measured to analyze the static characteristics of oscillation. For the dynamic characteristics, the escape probability was measured to evaluate the magnitude and the frequency of fluctuations. These experimental results were analyzed theoretically by a stochastic model developed to analyze the mode-hopping phenomena. Theoretical and experimental results were in agreement. This theoretical model was also applied to experimental results also agreed with the experimental results. These results indicate that this theoretical model can be used for the systematic description of the mode stability of semiconductor lasers. >