Rapid Laser Forming (RLF) was a new and advanced manufacture technology which has been developed on the base of combining high-power laser cladding technology with rapid prototyping to realize net shape forming of high performance dense metal components without dies. However, such defect as crack, pore, oxidation and ill bonding between cladding layers might appear in RLF parts if the process parameters controlled improperly. And crack was a kind of defect, which was the most familiar and companied with the largest brisance. The forming mechanism of the crack producing in the process of laser forming some alloy powders has been studied deeply in the paper through micro measurement and analysis methods. The results showed that the cracking behavior was different with different cladding materials. For the materials such as nickel-base self-fused alloys the cracking of laser cladding belonged to the cold crack domain. It was the result of synergistic effect of low ductilitv of such a11ovs and thermal stress inducing during laser cladding．On the other hand．for the materials 3S austenite stainless steel etc．the cracking of laser cladding belonged tO the hot crack domain． It was m ainly caused by the separation of the liquid film s in the grain boundary under the effect of thermal stress existed in the cladding which was producing in RIF.