The laser ultrasonic method has gained increasing visibility in non-destructive materials testing during recent years. By introducing and detecting broadband ultrasonic waves into and of metals in a contactless manner using laser radiation, it is a methodology that can be applied inline for quality inspection on an industrial scale, even in harsh environments and at high temperatures. However, apart from being a demanded method in industry, a laser-ultrasonic system also holds great potential for the in-situ analysis of the material’s microstructure for R&D. By selectively analyzing the propagation mode, direction and velocity of the acoustic waves, as well as determining the attenuation characteristics, various material properties can be characterized. Especially the coupling to heating and forming simulators allows the determination of these material properties during specific thermal and thermomechanical processes. Attenuation of acoustic waves by scattering effects at grain boundaries can be used to analyse the grain size. This effect was utilized in this study to measure the grain size evolution of Alloy 718 during heat treatments at 1050°C for different durations. For this purpose, a laser-ultrasonic system was combined with a dilatometer in which heat treatments were performed while the ultrasonic signals were recorded in-situ. After an appropriate model calibration, the results of the in-situ measurement could be compared and verified with optical microscopic images of the microstructure.