To investigate the flexural performance of unreinforced ultra high performance concrete （UHPC） one-way slabs and square slabs， experimental studies were conducted on the slabs under locally applied loads at mid-span， respectively. Based on the test results from the present study and other literature， the tensile constitutive model of UHPC was proposed in which the influence of the characteristic parameters of steel fiber was included. Through the nonlinear sectional analysis on the slabs， an equation was developed for determining the reduction factor k of equivalent uniform stress in the tensile zone of the unreinforced UHPC slabs at the ultimate. According to the test and analysis results， a simplified approach for calculating the flexural capacity of unreinforced UHPC one-way and square slabs was presented. The results show that all the unreinforced UHPC one-way and square slabs experienced a tension failure governed by the tensile properties of the UHPC. Owing to the enhancement by the steel fibers in UHPC， the unreinforced UHPC slabs exhibited a ductile failure mode with significantly greater loads and deformations at the ultimate state as compared to those at the initial cracking state. Because the plasticity of UHPC in tension is not enough to ensure the formation of a fully plastic mechanism in those unreinforced UHPC square slabs， the upper bound solution from the yield line theory is unsuitable to predict the flexural capacity of the slabs. However， the lower-bound one from the static method of limit analysis can give better predictive results with a safer margin， and the feasibility of the developed simplified approaches for predicting the flexural capacity of unreinforced UHPC one-way and square slabs are verified well by the test results.