The present invention provides a thermoplastic resin composition improved in stability at the time of processing which comprises (a) 100 parts by weight of a polyphenylene ether resin and (b) 0.001-15 parts by weight of at least one 4-amino-2,2,6,6-tetramethylpiperidyl compound represented by the following formula (I) or (II): ##STR1## wherein R.sub.1 represents a hydrogen atom, an oxygen atom (oxy radical), a hydroxyl group, an alkyl group, a cycloalkyl group, an allyl group, a benzyl group, an aryl group, an alkanoyl group, an alkenoyl group, an alkyloxy group or a cycloalkyloxy group;R.sub.2 represents a hydrogen atom, an alkyl group or ##STR2## where X represents a direct bond, --O-- or --NH--, R.sub.4 represents a direct bond, a straight or branched alkylene group, an alkylidene group or an arylene group, R.sub.5 represents a hydrogen atom, an alkyl group, an aryl group, ##STR3## n represents an integer of 1 to 4, p represents an integer of 2 to 6, andR.sub.3 represents a hydrogen atom or an n valent organic group.