Coke strength is mainly determined by pores and cracks that cause fracture of coke. In this study, connected pores that were considered to cause fracture of coke were investigated. In order to evaluate connected pores quantitatively, low roundness pores (whose roundness were below 0.2) were measured by image analysis technique of microscopic photographs of cokes. The relationship between the amount of low roundness pores and coke strength (DI^150₆) showed a good correlation. It is thought that the amount of low roundness pores is one of the factors determining coke strength.