Branched alkyl end groups of poly(methyl methacrylate) (PMMA) polymerized radically with 2,2’-aiobis(2,4,4-trimethylpentane) as an initiator were characterized by pyrolysis-gas chromatography (Py-GC). On the resulting pyrogram at 540 degrees C, characteristic products formed from the end group moiety due to the initiator, such as isobutane, isobutene, and so on, were clearly separated from those from the main chain. Then number-average molecular weight (M(n)) of PMMA was determined by the ratio of the relative intensity of these peaks due to the end group and the main chain. After simple correction using a reference PMMA sample having different end groups, M(n) values estimated by Py-GC agreed well with those obtained by size exclusion chromatography. Furthermore, determination of end groups well supported the assumption that disproportionation was dominant in termination in this polymerization system of MMA.