The macroscopic mechanical properties like yield-strength, ductility or hardness play an important role during the steel production and the design of new steel grades. The austenite grain size is an important parameter, which influences the final microstructure and the properties of a material. When developing grain growth evolution models, typically many samples have to be treated thermally and micrographs have to be prepared ex-situ. To reduce the time expenditure of this procedure we carried out in-situ laser ultrasound measurements of austenitic grain growth in plain carbon steel (AISI 1045). A thermomechanical simulator of the type Linseis L78/RITA has been upgraded with a laser ultrasound measurement system, which enables the continuous and contactless determination of the austenite mean grain size during a thermal cycle. In this work we will show the calibration workflow and grain size results by a new attenuation model for plain carbon steel. In-situ laser ultrasound measurement data is compared with several micrographs defined at supporting points along a specified temperature program to corroborate the findings.