Mechanical properties and microstructure of weld joint of SA 738 Gr.B with as welded and PWHT(post weld heat treatment)by SMAW(shielding metal arc welding)and GMAW(gas metal arc welding)are analyzed,it shows increasing welding heat input decreases impact properties of welded joint. During PWHT,weld undergoes grain recovery and recrystallization,meanwhile,inclusions in grain gather for weld and HAZ(heat affected zone) for SMAW at grain boundary,which decrease binding energy for grain boundary,it results in decreasing impact properties of weld and HAZ.