A method to measure the depth of open cracks in steel by laser ultrasound was developed. Surface waves were measured using a specimen with a slant slit whose depth varied continuously from 0 to 5 mm. The results showed that the time delay of the transmitted wave can be used to monitor the crack depth. When the specimen temperature was varied, the time delay varied because of the change in the propagation speed of the surface wave. In order to remove the effect of temperature change, a method using two surface waves was developed. The time delay of the reference wave, which only includes the effect of temperature change, was used to correct the time delay of the transmitted wave, which includes the effects of temperature change and crack depth. This method was shown to be effective in eliminating the effect of temperature change in the range from 25°C to 500°C