The fitness of Poisson distribution on the number of internal cracks and the fitness of log-normal distribution on the length of cracks was discussed. Concrete specimens were observed for internal cracking by means of digital microscope after they were subjected to several types of cyclic loading. The agreement of shapes between expected distribution and observed histogram was verified. The expedience of applying probability distribution to the number as well as length of cracks was substantiated. The research also indicated that the consideration for the number is more important than for the length of cracks. This paper is limited to work on internal cracks observed on the surface cut perpendicular to the direction of the compressive loading.
