Abstract: Using the hydraulically powered welding equipment developed by Tianjin University, underwater friction stitch welding of DH36 steel was conducted. The microstructures and mechanical properties (hardness and tensile property) of the weld have been investigated. The microstructure of the weld and the hardness distribution were heterogeneous. The overall microstructure of the weld metal consisted of a volume of lath martensite and bainite. The highest hardness value was 489.3 HV10, which located at the upper part of friction stitch weld. The hardness levels of the upper region of the stitch weld were higher than those of the lower region, and hardness levels of the last weld in the stitch sequence were higher than those of the first two welds. Stitch welds with higher welding axis forces showed better ultimate tensile strength than those with lower ones when the other conditions kept constant. And the tensile strength of welds in air was better than that of welds in water.