Plain woven C/SiC composites have been widely applied in engineering thermal protective structures due to their excellent mechanical and physical properties. Plain woven C/SiC orthotropic composites are composed of carbon fibers, and in-yarn and out-of-yarn SiC matrixes with small air pores. Experimental studies on the effective thermal conductivities of plain woven C/SiC composites are rare due to their complex structure and costly processing technology. The effects of fiber volume fraction and porosity on the effective thermal conductivities can be revealed by numerical approach. In the present study, a finite volume numerical model at three scale levels is proposed on the basis of 3D geometric reconstructions with in yarn and out-of-yarn porosities. The thermal conductivities of SiC matrix, woven yarn, and plain woven C/SiC composites are predicted using up-scaling approach. The proposed model is validated using available analytical and experimental results. The thermal conductivities of plain woven C/SiC composites decrease with the increment in carbon fiber volume fraction, and in-yarn and out-of-yarn porosities. The effect of out-of-yarn porosity on the thermal conductivities of the composites plays a dominant role, However, the occurrence of in-yarn porosity can lead to maximum decreases of 3.1% and 5.6% for in-plane and out-of-plane thermal conductivities respectively compared with the case without in-yarn porosity. The present work can provide a scientific guidance for the thermal design of plain woven C/SiC composites. (C) 2019 Elsevier Ltd. All rights reserved.