The solidification sequence and microstructure evolution during solidification process of two 316LN stainless steels with different compositions under different cooling rates were in situ observed with confocal scanning laser microscope. The results show that 316LN solidifies with primary austenite or primary delta ferrite when the cooling rate is small in the range of conventional casting process, depending on the value of Cr-eq/Ni-eq which are calculated by Hammar and Svensson equations. As the cooling rate increases in the range of 0-100 A degrees C s(-1), the solidification sequences do not change, but both the dendrite arm spacing and the mean free path between delta ferrite decrease. In addition, concomitant with the variations of chemical composition in delta ferrite and austenite are the shape transformation of interdendritic delta ferrite from island-like to lacy-like and the coarsening of dendrite delta ferrite with cooling rate increasing. The mechanism of three-phase reaction in 316LN with different compositions, i.e., eutectic reaction or peritectic reaction, was analyzed. The bigger diffusivities of Cr and Ni in primary delta ferrite than that in primary austenite and the positions of alloys in phase diagram were thought to be the main reasons for the difference in type of the reaction.