Syndiotactic poly­(methyl methacrylate) (st-PMMA) is known to form an inclusion complex with fullerenes in toluene. We investigated in detail the influence of the primary structure of st-PMMA, such as the molecular weight and stereoregularity, on the inclusion complex formation with C60. The formation of an inclusion complex with C60 was accompanied by gelation that occurred in toluene by using st-PMMAs with an rr content of 89% and an M n of over 7.8 × 103 in the polymer, with C60 concentrations of 10 and 2 mg/mL, while no gelation was observed when using st-PMMA with an M n of 4.6 × 103. The st-PMMA gelation ratio and the C60 encapsulation ratio increased with the increasing molecular weight and became constant at an M n of over 2.9 × 104. The guest C60 molecule also had a large influence on the formation of the helical structure of st-PMMA. To obtain a gel quantitatively without fullerene, it was necessary to use st-PMMA with a high M n of 5.8 × 105, whereas in the presence of C60, st-PMMA with an M n of 2.9 × 104 was sufficient. The influence of stereoregularity on the inclusion complex formation was also investigated using st-PMMA with a sufficiently high molecular weight (M n > 3.0 × 105) and different rr contents. Even when the rr content of st-PMMA decreased from 97 to 89%, the C60 encapsulation ratio did not decrease. Since the main chain of st-PMMA with an rr content of 89% incorporates approximately one defect (m-diad) per helical turn, it was concluded that the helical structure of the polymer chains is maintained when the defects are at this level.