Abstract: The heavy steel plates with narrow gap groove were welded by gas metal arc welding under three heat inputs,and the effects of heat input on microstructure and properties of the weld metal were investigated by means of optical microscopy,scanning electron microscopy with energy dispersive spectroscopy,and transmission electron microscopy.The results indicate that the microstructure of the three weld metal consist predominantly of lath martensite,carbide free bainite,M-A constituent and retained austenite.The amount of martensite decreases with the increasing of the heat input.As the heat input increases,bainite nucleation rate decreases,resulting in coarser bainite,and C diffuses the farther distance,resulting in variation of retained austenite morphology from strip to block.The strength of weld metal drops with the increasing of heat input,while the toughness is not sensitive to heat input.