In order to clarify the factors related to coke strength degradation, relationship between coke strength and pore structure is quantitatively investigated. The tensile strength of coke is measured by diametral-compression test and analyzed by Weibull plot. Pore structure is analyzed with wide range(10 mm×10 mm) and high resolution(2.43 μm/pixel) photographs which are acquired by combining approximately 20 photographs. Absolute maximum pore length, pore area ratio and pore roundness are measured by image analysis of the photographs. By comparison of strength and absolute maximum pore length in coke, strength is degraded with an increase in area ratio of pores over 1000 μm in absolute maximum length. From image analysis, it is found that the pore roundness decreases with an increase in absolute maximum length. The length of 1000 μm corresponds with the critical crack length calculated by Griffith equation in scale.