An FEM (Finite Element Method) numerical approach of predicting the effective thermal conductivities of plain woven composites is presented in this paper. Three reducing-size unit cells are formulated by using different symmetries exhibited in the composite, including translational, reflectional and rotational symmetries. Corresponding thermal boundary conditions are derived and validated by the numerical results of the same problem of different unit cells. Thermal conductivities of the matrix with porosity and the woven yarns are calculated first, and then used as input data to numerically predict the effective thermal conductivities of plain woven composite. The influences of porosity and fiber volume fraction on effective thermal conductivities of studied composites are clarified. (C) 2015 Elsevier Ltd. All rights reserved.