A process for modification of a polymer comprising modifying a polymer (A) capable of decomposing by carbon radicals present in the molecule thereof using a compound (B) having a nitroxide radical in the molecule thereof and a radical initiator (C) in a ratio of (B)/(C) (molar ratio) of at least 0.7, whereby organic groups derived from the nitroxide radicals are introduced into the polymer and a modified unhalogenated butyl rubber obtained therefrom.