The glass-transition behaviors of the nano-composites comprised of poly(methyl methacrylate) (PMMA) and multiwalled carbon nanotubes (MWCNTs) were investigated. The average glass-transition temperatures (T-g) of PMMA/CNT nanocomposites strongly depend on the nanocomposite structure or the orientation of CNTs in composites. The coexistence of the polymer layer with high mobility at the free surface of PMMA coating on CNT sidewalls and low-mobility PMMA chains in the interfacial zone locally in contact with CNTs leads to the local heterogeneity of polymer dynamics. In the PMMA/CNT nanocomposite with vertically aligned and densified CNT arrays (DCNTA), the PMMA chains with reduced dynamics in the interfacial zone near CNT sidewalls dominate the average T-g and lead to an increase in T-g. In the PMMA/CNT nanocomposite with randomly distributed CNTs (RCNT), a polymer layer with high mobility at the free surface of PMMA coating on CNT sidewalls causes a decrease in average T-g. The PMMA/CNT nanocomposites also exhibit a broader glass transition.