In the production with thin slab casting a serious problem of insufficient grain refinement and microstructural inhomogeneity has been found. In this paper attention has been paid to the austenite grain size changes in thin slab of a high strength Nb microalloyed steel under as-cast conditions and after first rolling pass. For comparison, the conventional thick slab was also investigated. It was found that although as-cast thin slab shows a smaller average austenite grain size than that of as cast thick slab; the latter after reheating shows a much finer average austenite grain size. The first rolling pass at high temperature and with heavy reduction causes rapid recrystallization that contributes to austenite grain refinement.