The effects of stereochemical structure of MMA oligomers on the fragmentation during ionization process in electron-impact ionization and chemical ionization mass spectrometry were studied using the oligomers prepared by stereospecific living oligomerization. The MMA oligomers having t-C4H9- group at the left end and methine proton at the right end showed strong tacticity dependence in their fragmentation process. Fragmentation of MMA tetramer having one head-to-head linkage in the middle of the chain was also studied.