This study aims to quantify the effect of physical aging progression on viscoelastic property in carbon/epoxy composites. Understanding viscoelastic behavior of carbon fiber reinforced plastics (CFRPs) has become an essential issue for further application of CFRPs to aerospace structures, which require high dimensional accuracy. In this research, the viscoelastic property was evaluated by quasi-static tensile tests in transverse direction for unidirectional laminates. The progression time of physical aging versus viscoelastic parameters was obtained. It was found that the viscoelastic behavior becomes less significant as the physical aging progresses. Considering the quantitative result, creep compliance function, which is conventionally difficult to predict due to the unknown physical aging effect, was predicted as functions of creep time and preceding aging time. This prediction was verified by actually performing a long-term creep test in transverse direction for unidirectional composites. Creep test result and the calculated results using viscoelastic parameters obtained from quasi-static tensile tests showed similar results up to the aging time given to the specimen before the test, but they showed different trends along with time progression. By taking into account the progression of physical aging, the calculated lines are likely to show better correspondence with the creep curves.