Abstract: The crack in concrete block wall is a new problem for the new wallmaterials. At First, based on engineering investigations, causes andpositions of cracks in concrete block wall are studied, and temperature crackand dry shrink crack are two main kinds of cracks. Secondly, the temperaturecrack stress is calculated by temperature difference in an existing building.Thirdly, the mechanism of dry shrink crack is expounded by Kelvin-equation andthe dry shrink crack stress are also analyzed. In the end, causes ofcracks in concrete block wall are explained.