Abstract: The influences of laser power, welding velocity, defocusing distance and flow rate of side-blow shielding gas on shape of weld, and weld penetration of laser deep penetration welding of dissimilar metal K418 and 42CrMo were experimentally investigated using continuous wave (CW) Nd:YAG laser.Mechanism of hot crack formation of weld zone was analyzed.Results show that lack fusion between weld and 42CrMo interface is observed due to difference of theirs physical and chemical properities.Offset of laser beam toward 42CrMo can abate lack fusion between weld and 42CrMo interface.The serrate shape is observed between weld seam and K418 interface due to not full convection heat transfer.Weld penetration can be increased by optimization flow rate of side-flow shielding gas and defocusing distance.The main cause of hot crack in the weld zone of dissimilar metal of K418 and 42CrMo laser welding is the segregation of the element and the forming of low-melting phases over grain boundaries.