We present a three-dimensional phase-field simulation of membrane porous structure formation via spinoclal decomposition by the thermally induced phase separation (TIPS) method. The free energy of the polymer solution system was described based on the Flory-Huggins theory, and the mobility was calculated from the solvent self-diffusion coefficients estimated by the free-volume theory of Vrentas and Duta. We explored the effects of initial polymer concentration, quench temperature, and gradient energy parameter on the membrane morphology evolution. The initial domain size increased with the decreasing driving force for the spinoclal decomposition and the increasing contribution of the gradient energy to the total energy. The morphology evolution during the intermediate and late stage of spinodal decomposition also depended on the membrane morphology formed at the initial stage, such as bicontinuous or droplet structures. We also simulated the membrane formation under the influence of a polymer concentration gradient in the TIPS process. The polymer concentration gradient led to the formation of membranes with anisotropic pore structures. (C) 2015 Elsevier B.V. All rights reserved