A process for producing a terminal-modified polyphenylene ether represented by formula (III): ##STR1## wherein Q.sup.1 represents a halogen atom, a primary of secondary alkyl group, a phenyl group, an aminoalkyl group, a hydrocarbon oxy group, or a halo-hydrocarbon oxy group; Q.sup.2 represents a hydrogen atom, a halogen atom, a primary or secondary alkyl group, a phenyl group, a haloalkyl group, a hydrocarbon oxy group, or a halo-hydrocarbon oxy group; n represents an integer of at least 10; X represents an oxygen atom or a nitrogen atom;R.sup.1 represents an alkylene group having from 1 to 12 carbon atoms; R.sup.2 and R.sup.3 each represents a hydrocarbon group having from 1 to 6 carbon atoms; s represents 1 when X is an oxygen atom, or 2 when X is a nitrogen atom; and t represents an integer of from from 1 to 3,which comprises reacting a polyphenylene ether represented by formula (I): ##STR2## wherein Q.sup.1, Q.sup.2, and n are as defined above, with a compound represented by formula (II): ##STR3## wherein X, R.sup.1, R.sup.2, R.sup.3, s, and t are as defined above; a process for producing an alkoxysilylated polyphenylene ether represented by formula (V): ##STR4## wherein Q.sup.1, Q.sup.2, and n are as defined above; R.sup.11 represents a bond or an alkylene group having from 1 to 12 carbon atoms; R.sup.12 and R.sup.13 each represents a hydrocarbon group having from 1 to 6 carbon atoms; and m represents an integer of from 1 to 3, which comprises reacting a polyphenylene ether represented by formula (I) shown above with a compound represented by formula (IV): ##STR5## wherein R.sup.11, R.sup.12, R.sup.13, and m are as defined above; and Y represents a halogen atom;and a thermoplastic resin composition comprising the modified polyphenylene ether of formula (III) or (IV) with a saturated polyester, a polyamide, or a modified olefin resin.