To understand the mechanism of the viscous slowdown in supercooled liquids that is responsible for glass transition, we investigate the interrelation between glass transition temperature T g , fragility, and cooperativity in segmental dynamics. Polymeric glass-formers having a similar chemical structure are expected to exhibit clear correlations between the above parameters. In this paper, polystyrene (PS) derivatives possessing various para-substituents are studied using calorimetry in regard to the fragility parameter m, dynamic length scale ξ, and the number of cooperatively rearranging segments N CRR at T g. Positive correlations were revealed for both N CRR (T g) vs. m and ξ 3 (T g) vs. m. Both fragility and cooperativity were found to increase as the bulkiness of the substituent increases. Wide-angle X-ray scattering measurements revealed that the structural correlation between backbone chains is reduced as the bulkiness increases. This may be responsible for the reduced cooperativity. In contrast, for poly(methacrylic acid ester)s, the relations between the above parameters appeared to be less clear. The clear trend observed for the PS system may be due to the rigidity of the phenylene unit, through which the substituent directly affects the backbone dynamics. The estimated activation energy per segment increased consistently with an increase in the para-substituent's bulkiness.