Abstract: This paper presents a simplified method for predicting the deflection of reinforced ultra-high toughness cementitious composite (RUHTCC) beam under the service state, based on the parameter of the effective inertia moment of cross section. Different from reinforced concrete beam, the tension capacity of UHTCC in tension zone cannot be ignored. The theoretical analysis further indicates that the neutral axis obtained based on the equilibrium of area moment is different from that calculated from equilibrium of internal force with the assumption of linear distribution of compressive stress, while the latter accords with the physical meaning of neutral axis. By taking into account the nonlinear tensile and compressive constitutive models and the secant modulus corresponding to the strain of the extreme compressive fiber, the equations for neutral axis depth and inertia moment of full cracked section are obtained based on the equilibrium of internal force. Furthermore, for RUHTCC beam, the coefficient m in equation for effective inertia moment of section is revised to be a function with the parameter of longitudinal reinforcement ratio. The predicted load-deflection curve before yielding based on the proposed simplified calculation method shows good consistence with the experimental results.