The tensile strength of coke was studied to obtain a better understanding of coke strength and its relation to coal properties. The tensile strength was examined by considering coke as a porous material with two influential factors, namely, the matrix strength and the porous structure, which were estimated from the measured tensile strength and porosity. The coke carbon matrix was evaluated in the light of the structure of the coke carbon estimated using X-ray diffraction on the basis of the concept of graphitic and non-graphitic carbon. It was found that the coke matrix strength becomes stronger as the coke carbon structure approaches that of non-graphitic carbon. The porous structure was governed by the coking properties, in this case, the maximum fluidity and the swelling number. As regards the factors governing the coke porous structure, determined by means of image analysis and optical microscopy, it was demonstrated that a poor porous structure was associated with a wide pore size distribution caused mainly by a high proportion of the pores of length < 20 mu m, and a rough surface of the pore periphery, which provides potential stress concentration points. The strength anisotropy, which is considered to affect the route of a crack path in lump coke, was also related to pore orientation and the quality of the porous structure.