Underwater friction stir welding (UFSW) has been conducted on 5052-aluminum alloy in of 6-mm thickness plate at various tool rotational speeds, welding speeds and type of welding tools. The welding speed varied from 50 mm/min to 150 mm/min while the rotational speed varied from 500 rpm to 2000 rpm with three different type of welding tools viz., square, tapered cylindrical and taper threaded cylindrical. The exploration discussed the effect of different parameters on the microstructural features of the weld joint. The results discovered that weld joints produced by the taper threaded cylindrical tool display defect free stir zones due to well composed of heat generation and balanced material flow. Both the heat input and the material deformation dominated the grain size in weld joint when using square tool. The material deformation essentially dominated the grain size via tapered cylindrical welding tool, and the heat input dominated the grain size by means of taper threaded cylindrical welding tool. © 2018 Author(s).