In the glass-to-rubber transition zone over a wide temperature range, we simultaneously measured the complex Youngs modulus, E^*(ω), and the complex strain-optical coefficient, O^*(ω), of three poly(carbonate-g-styrene) copolymers. Two of them include 5 and 10 wt% of maleic anhydride (MAH) in their grafted styrene, respectively. We compared the relaxation behaviors in E^*(ω) and O^*(ω) of the copolymers in view of the compatibility of their component polymers. The E^*(ω) of three copolymers displayed typical glass-transition behavior of amorphous polymers and apprehensible differences were not appreciated between the copolymers. For three copolymers, minute dispersion was observed in the lower frequency region of dominant tan6 peaks arising from minute phase separation of the components. With increasing MAH content in the copolymers, it was absorbed into the main α-distribution, which indicated improved cooperativity of chain relaxation between dissimilar polymer species. Contrasting with the similar mechanical properties, the optical properties, O^*(ω), reflected a great sensitivity to the minute variation of chain relaxation. The optical relaxation spectra of three copolymers showed definite difference in quality, indicating unlike optical relaxation mechanisms. The characteristic differences among the copolymers were simulated by assuming a simple additivity rule of the O^*(ω) of component polymers. The compatibility of the copolymers was discussed in light of a fractional contribution parameter, F_ps, which was derived from the simulation.