The forming mechanism for the crack produced in the process of laser forming 316L stainless steel powder alloy has been studied in the paper by micro measurements. The results show that the cladding cracks were formed in grain boundary with dendritic structure commonly and presented a characteristic of cracking along grains. There was obvious oxidation color on the fracture section of the cracks. SEM photos show that the orientation and profile of dendrite grains are distinct and the dendrites are rather round on the crack surface. All these show that the cracks were formed at high temperature. The crack formed in the cladding layers is the solidification crack, which belongs to the domain of hot crack, mainly due to the separation of the liquid films in the grain boundary under the action of tension stress existed in the cladding layers which was produced in RLF process.