This paper explains the mechanism of nonlinear behavior of expansive mortar under tensile stress. Through flexural and uni-axial cyclic loading including unloading process, it is proved that tensile stiffness of expansive mortar during unloading gradually decreases when maximum tensile stress applied increases, and that expansive mortar shows residual tensile strain after unloading. Based on these experimental facts, a new concept is proposed to rationally explain the nonlinear behaviors peculiar to expansive mortar in tension. It is shown that not only increment of tensile strength due to prestress but also large deformability before cracking greatly contributes to the cracking resistance of chemically prestressed members.