The present work theoretically and quantitatively investigates the geometrical effects of the electrode containing a coolant hole on heat transfer or temperature gradient, and solidification rate responsible for microstructure of the nugget during resistance spot welding. Resistance spot welding has been widely used in joining thin and small-sized workpieces in automobile, aerospace, and different manufacturing fields. This model adopted from previous work realistically accounts for transient magneto-fluid mechanics, heat and species transport, and bulk resistance in workpiece and electrode, and film and constriction resistances at contact interfaces. The computed results show the geometrical effects of the electrode containing a coolant hole on heat fluxes, and nugget growth and solidification rates in different directions. In view of the smaller heat flux and higher solidification rate in radial direction than those in axial direction, equiaxed grains due to a lower morphology parameter are usually observed in the central region of the weld nugget. Morphological parameter in both directions decreases or equiaxed grains readily occur, for example, if the face radius and truncated length of the electrode increase. Fine spacings of the primary and secondary dendrite arms resulting from enhanced cooling rate can be achieved by maintaining the coolant hole close to the electrode face. Different microstructures of the weld nugget therefore can be controlled via designing the shapes of the electrode containing coolant hole. (C) 2014 Elsevier Ltd. All rights reserved.