This study aims to quantify an effect of physical ageing progression on viscoelastic property in carbon/epoxy composites.The viscoelastic property is evaluated by quasi-static tensile tests in transverse direction for the unidirectional composites.The progression time of physical ageing vs.viscoelastic parameter is obtained.It is found that the viscoelastic behavior becomes less significant as the physical ageing progresses.Considering the quantitative result, an accurate creep compliance function, which is difficult to be predicted due to the unknown physical ageing effect conventionally, is verified to be predictable as functions of creep time and preceding ageing time by comparison with experimental results regarding actual creep test.