Micro cracks with average widths ranging from 20μm to 88μm were induced by three points bending on two types of reinforced concrete C30 and C50. The process of water penetration into these micro cracked concrete were studied experimentally by means of neutron radiography. The spatial water distributions in cracked concrete during water penetration were calculated quantitatively. The results indicate that cracks provide preferential paths for water penetration into cracked concrete even though the crack width is only 20μm. Water penetrated into the fine cracks once contacted the cracked concrete, it will be immediately followed by migrating into the adjacent region from two sides of the cracks horizontally and into the region behind the cracks end vertically. When crack width is up to 82μm, which corresponds to the ratio of 039% between crack width and cover thickness, there is also water ingress existing in the interfaces between steel reinforcement and cement matrix. But the water content in this area is very low. The water profiles have peaks at the positions of cracks, where the profiles start decreasing from both sides of cracks symmetrically.