Internal force of a concrete member duo to temperature variation is related to stiffness. A reduction of stiffness occurs and the magnitude of the internal forces drops from the values existing before cracking when cracking occurred due to temperature variation. Internal force of axial tension member due to temperature variation developed in a jagged style with cracking. Calculation method of internal force and stiffness reduction coefficient of axial tension member due to temperature variation was presented accounting for cracking. Given example shows stiffness reduction coefficient has a close relationship with number of cracks and steel ratio. Member with high steel ratio subjected to the same temperature variation has more cracks and smaller crack width.