The effect of postweld heat treatment (PWHT) on microstructure and properties of the heat affected zone (HAZ) in ASTM4130 steel fabricated by single-pass welding was investigated by weld thermal simulation test, optical microscope, transmission electron microscopy and scanning electron microscope. The results show that the subcritical HAZ (SCHAZ) toughness slightly decreases due to the aggregation and growth of carbides. Other HAZ toughness is significantly improved showing that the 650°C/1.5h PWHT is effective. The toughness improvement of the fine-grained HAZ (FGHAZ) and the intercritical HAZ (ICHAZ) is chiefly resulted from the complete decomposition of martensite and M-A constituent. Meanwhile, the supersaturation reduction of non-equilibrium microstructure and the relief of residual stress effectively improve CGHAZ toughness.