Abstract: Solidification processes of AISI304 austenitic stainless steel at cooling rates of 0.05℃·s^-1 and 3.0℃·s^-1 were observed in situ by confocal laser scanning microscopy. When the cooling rate is 0. 05℃·s^-1 the solidification front is cellular morphology and the solidification mode belongs to FA mode. B ferrite appears firstly in liquid steel, and then γ phase precipitates prior at B-grain boundaries at 1 448.9℃ by means of the periteetic reaction (L+δ→γ). At 1 431.3℃ the liquid steel disappears. Finally the B ferrite transforms to γ phase by solid diffusion, and the retained ferrite which enriches in Cr and depletes in Ni locates between the cellular dendrites. When the cooling rate is 3.0℃·s^-1, the solidification interface morphology is dendritic. The peritectic reaction takes place at interfaces between B-grains and the remained liquid at 1 346.4℃, and the retained ferrite locates in the dendrite arm. Compared to 0.05℃·s^-1, the degree of enrichment in Cr and depletion in Ni in the residual ferrite decreases but the amount of them increases.