Abstract: The cracking cause of water-cooling furnace roller in horizontal continuous annealing furnace of a strip steel hot continuous galvanizing line was studied by macroscopic and microscopic morphology observation, chemical composition analysis, microstructure observation and hardness test. The results show that the composition of the water-cooling furnace roller sleeve contained no niobium element and a low content of nickel element, resulting in the formation of network rich chromiun carbide precipitates. The scaling of roller sleeve chamber and the poor water-cooling effect caused the local overheating of water-cooling furnace roller; then the granular alloy carbides precipitated, and network rich chromiun carbide of roller sleeve at working side coarsened resulting in the increase of brittleness of roller sleeve at working side. With the deposition of scale, the poor bearing rotation or even rotating-jam in water-cooling furnace happened. Under the thermal stress and torsion force, the crack originated from the network rich chromiun carbide, and gradually extended into the matrix along the network carbide, which finally caused the cracking of the water-cooling furnace roller.