Abstract: The influences of welding heat input on weld of laser welding of K418 and 42CrMo dissimilar metal were experimentally investigated using continue wave Nd:YAG laser of output power 3 kW.Microstructure of the welded joint was studyed by optical microscope, scanning electron microscope, X-ray diffraction, and energy dispersive spectrometer.Mechanical properties of the weld were evaluated by hardness and tensile strength test. Results show that weld penetration of keyhole welding mode is larger than of heat conduction welding mode with constant linear heat input.Particle of rich Nb, Ti and Mo and deleted Fe and Ni and needle of rich Nb and Ti are observed in the fusion zone.The tensile strength of weld is higher than that of base metal of 42CrMo by optimizing laser welding parameters.