Abstract: The closure and modification of a real crack in a black glass sample caused by laser irradiation were observed by laser ultrasonic method. The ultrasonic generated by point pulsed laser was detected using optical deflection method. The crack was irradiated by a continuous heating beam and closed gradually by increasing the power of the heating beam. At the same position of the crack, the power of the heating beam was increased gradually and the peak-to-peak value of transmitted surface acoustic wave and longitudinal wave mode converted by surface acoustic wave was recorded at each heating power both when the crack was heated and cooled after heating. The whole process is repeated three times. Experiment results show that great differences of the peak-to-peak values exist between the crack with heating and without heating. Also, the significant difference of the experiment result between the first time and the last two times was observed, with the latter two results whereas being very similar. The results indicate that irreversible modification could happen to the crack during heating and the modification becomes unnoticeable after several times of heating because the crack become more stable.