The interphase between fiber and resin in a composite material has a three-dimensional region. The interphase has an important role, namely, supports the stress transmission between fiber and resin. However the influence of this interphase on the mechanical properties of the whole laminated composite has not been clarified. In this study, the more flexible interphase than the matrix resin was proposed. The fabrication method of unidirectional CFRP laminates with flexible fiber/matrix interphase has been established. The influences of the flexible interphase thickness on the flexural modulus and strength were experimentally investigated. Also the relationship between the interphase rigidity and the flexural properties were found out quantitatively. Based on these experimental results, a new designing parameter of flexible interphase was introduced for the improvement of flexural properties of unidirectional CFRP laminates.