A method of manufacturing a golf ball is disclosed. A cover is molded through use of either a thermoplastic resin having a group capable of reacting with an isocyanate group or a thermoplastic resin containing a compound having two or more groups capable of reacting with an isocyanate group. Subsequently, a polyisocyanate compound is caused to permeate into the surface layer of the cover to thereby cause the reaction between the polyisocyanate compound and the group(s) capable of reacting with the isocyanate group. As a result, a modified layer having excellent properties is formed at the surface of the cover formed from thermoplastic resin. In this case, a non-yellowing polyurethane resin is preferably used as the thermoplastic resin having a group capable of reacting with an isocyanate group, and 4,4-diphenylmethane diisocyanate (MDI) is preferably used as the polyisocyanate compound.