The present paper is concerned with prediction of the overall thermal conductivity of carbon fiber polymer matrix composites during volumetric ablation due to pyrolytic thermal decomposition. Volumetric ablation describes the loss of mass process during which the overall volume of the composite material does not change in the presence of thermal decomposition (pyrolysis) of the polymer matrix. Thermal decomposition caused by heating leads to formation of solid pyrolytic and gaseous phases in the composite material. The volume fractions of char, gas, and polymer phases are temperature-dependent and are obtained in the present work from the Arrhenius-type equation describing decomposition of the polymer matrix. The microstructure generation algorithms are developed to create highly packed microstructures consisting of circular (representing fibers) and elliptical (representing pores) inclusions and to accommodate the growth of pores with temperature. The computational results for the overall thermal conductivity are obtained for the AS4/3501-6 composite in a temperature range up to 700 K.