In order to investigate the mechanical properties and constitutive relation of stress-strain for ULCC (ultra lightweight cement composite), four types of ULCC with different densities ranging from 1 250 kg/m³ to 1 550 kg/m³ and the corresponding compressive strengths ranging from 47.9 MPa to 70.0 MPa were developed in this paper. These ULCC consisted of microlightweight aggregates (cenospheres), cementing materials (cement and silica fume), concrete admixture (shrinkage reducing admixture and superplasticizer) and steel fiber with volume fraction of 1%. An experimental campaign including uniaxial compressive strength tests and uniaxial tensile strength tests was undertaken to investigate the uniaxial compressive and tensile mechanical properties, and the compressive strength, tensile strength, elastic modulus, poisson ratio, uniaxial compression stress-strain curves and uniaxial tension stress-strain curves were obtained. The results show that compressive strength, tensile strength, and elastic modulus all increased with the increasing of densities. The compressive strength and elastic modulus of ULCC were linearly related to densities. The segmented constitutive equations of ULCC under uniaxial compression and uniaxial tension were proposed based on the uniaxial compression stress-strain curves and uniaxial tension stress-strain curves obtained from the tests. The research results can provide theoretical basis for the design and nonlinear finite element calculation of ULCC structure.