A unified coupled multiscale mechano-diffusion-reaction model of environmental degradation of polymer matrix composite (PMC) and ceramic matrix composite (CMC) is developed. The unified multiscalemultiphysics model couples multiple physical processes at multiple scales, including oxygen diffusion, oxidation, and deformation. The salient feature of the unified multiscalemultiphysics model is its computational efficiency accomplished through a systematic model reduced carried out prior to nonlinear analysis. The model has been validated for PMR-15 reinforced carbon fiber composite and melt infiltrated CMC-NASA N24A material system.