A radiation curable polyurethane resin includes an ionic group, a polyalkylene oxide in a side chain thereof, and a (meth)acrylate or (meth)acrylamide having a hydroxyl functional group. The polyurethane resin is obtainable by reacting a polyester polyol, a polyether diol, a polyol containing an ionic group, a (meth)acrylate or (meth)acrylamide having a hydroxyl functional group, and a polyisocyanate. The polyester polyol is obtained by reacting a polycarboxylic acid and a polyol. The radiation curable polyurethane resin can be used as binder in an aqueous ink jet ink.