Desirable patterns formed by microphase separation in block copolymers make these materials attractive for miniaturization of functional devices in which formation of nanostructures is highly demanded. The microphase separation forms microdomains under three-dimensional confinement for each component and thus substantially changes physical properties such as structural relaxation of the polymer blocks. In this article, we report enthalpy relaxation and morphology evolution in polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) durin isothermal physical aging, measured by calorimetry and atomic force microscopy (AFM). Under nanoconfinement formed by microphase separation, the PS blocks have higher relaxation rate and amount of relaxed enthalpy compared with the corresponding homo-PS, while the PMMA blocks have lower relaxation rate and relaxed enthalpy than the homo-PMMA. The in situ morphology evolution shows that the characteristic distribution profiles of the AFM phase angle become lower and wider with increasing aging time for both the blocks, and the average distance between the central phase angles of the two blocks increase during aging. As large AFM phase angle of PMMA representing higher modulus, it demonstrates that the PS blocks under hard confinement exhibit greater enthalpy change with enhanced relaxation rate from the bulk. In contrast, PMMA blocks under soft confinement show the opposite trend.