Influences of compressive and tensile stresses on an electrical resistivity of carbon fiber in polyvinyl acetate polymer on bending test are studied as a basic research to develop high sensitive stress sensors. It is confirmed that a compressive stress of less than 0.3 GPa on bending test reversibly decreases the electrical resistivity of carbon fiber due to enhancement of the density of state of π bonding electron. A tensile stress of less than 0.3 GPa on bending test also decreases the electrical resistivity of carbon fiber, reversibly, because of decreasing the density of electron scattering sites.