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Due to the bridging effect of steel fibers in ultra-highperformance concrete (UHPC), reinforced UHPC beams exhibit much higher post-cracking stiffness and smaller deflection compared with conventional reinforced concrete beams. This paper attempts to develop a mechanics-based yet simplified method for determining the instantaneous deflection of reinforced UHPC beams at serviceability limit state. First, a validated numerical procedure is developed to predict the moment-curvature behavior of reinforced UHPC sections under serviceability conditions. Then, the tension stiffening in the beam is evaluated. It is found that the moment-curvature response of the UHPC section at the stabilized microcracking stage is almost parallel to that of the fully cracked section, with a nearly full tension-stiffening response. On this basis, a design equation for the effective moment of inertia is derived for calculating the deflection of …