Recently, high heat input welding have gained renewed engineering significance for improving welding efficiency and lowering construction cost. To achieve the desired microstructure and hence properties, it is inevitable to control the heat input because different heat input usually means various t8/5. In the current work, the effect of varying heat input, between 120, 210, 510 and 680 kJ/cm on the microstructure of EH36 shipbuilding steel has been investigated. Significant grain coarsening was observed in the heat affected zone (HAZ) of all the samples and it was found that the extent of grain coarsening in HAZ increased with increase in the heat input. Meanwhile, with the increase of heat input value, the number of fine inclusions (smaller than 1 μm) decreases and growth of inclusions is found to accelerate. Simultaneously, the nucleation points of acicular ferrite decreased.