The present invention relates to an industrially beneficial method of manufacturing golf balls which includes the steps of pre-preparing a masterbatch of an unsaturated carboxylic acid and/or a metal salt thereof by mixing an unsaturated carboxylic acid and/or a metal salt thereof with a rubber material, preparing a rubber composition that contains the rubber material by using the masterbatch, and employing a material obtained by molding the rubber composition under heat as a golf ball component. The masterbatch is composed of:
(A) from 20 to 100 wt % of a modified polybutadiene obtained by a modification reaction wherein a polybutadiene having a vinyl content of from 0 to 2%, a cis-1,4 bond content of at least 80% and an active end is modified at the active end with at least one type of alkoxysilane compound, and
(B) from 80 to 0 wt % of a diene rubber other than Ingredient A.
such that ingredients A and B are included in a combined amount of 100 wt %, and
(C) an unsaturated carboxylic acid and/or a metal salt thereof.