Molecular motion in an interfacial region of microdomains of polystyrene-block-poly(methyl acrylate) (PS-block-PMA) was studied by electron spin resonance (ESR) technique. The junction points between the blocks, which are located in the interfacial region, were labeled with stable nitroxide radicals. Mobility of the spin-labels reflected the dynamic environments in the interfacial region. The transition temperature of the motion of the spin-labels, T-5.0mT, at which the extreme separation width due to N-14 anisotropic hyperfine splitting is 5.0 mT, was estimated, and it reflects a glass transition of the region around the labels. The T-5.0mT of the PS-block-PMA labeled at the junction point was almost the mean value of those of the spin-labeled PS and PMA homopolymers, and the distribution of the motional correlation times (tau(c)) in the interfacial region was much broader than that in the homopolymers. These results are considered to be caused by the heterogeneous mixture of the each segment in the interfacial region at a certain length scale. The molecular weight strongly influenced the interfacial thickness and the segmental mobility and the width of the distribution of the tau(c) in the interfacial region as well as on the glass transition temperatures (T-g's) of the microdomains. It was revealed that the width of the distribution of the tau(c) depended on not the interfacial thickness so much as the difference between the mobilities of the block chains in the microdomains. On the other hand, extremely small effects of the overall composition and the morphology of the PS-block-PMA on the segmental mobility in the interfacial region were observed. From these results, it was considered that the dynamic environment in the interfacial region was strongly affected by the gradient of the segmental concentration in the interfacial region and the mobility of the block chains in the microdomains.