The presence and penetration of water and chemical compounds dissolved in water into reinforced concrete play a crucial role in deterioration of material and, hence, for the durability and service life of structures.The problem will be more acute once cracking has occurred which is usually inevitable in real structures due to mechanical, physical or chemical actions.The objective of the project is to study the behavior of water penetration into cracked reinforced concrete with fine cracks induced by mechanical load.Two types of reinforced concrete were tested: M1 with W/C ratio of 0.6 and grade of C30, and another mortar M2 with W/C ratio of 0.4 and grade C50.Cracks with average widths ranging from 20 μm to 130 μm induced by three-point bending were selected to perform water penetration test.The process of water penetration into these cracked reinforced concrete were followed visually by means of the technique of neutron radiography.The spatial water distributions in cracked concrete have been calculated quantitatively.It turned out that even very fine micro-cracks are immediately water filled whenever the surface comes in contact with liquid water.It was closely followed by further ingress of water front gradually into the fracture process zone behind the crack.In the meantime water migrated horizontally into the adjacent area surrounding the cracks.When crack width is below approx.50 μm in concrete M1, no water ingress can be found in the interfaces between steel bars and cement matrix.In concrete M2 this crack width is approx.130 μm.Fine cracks also provide preferential paths for water penetration into reinforced concrete, and thus may result in accelerated deterioration and reduced service life of structures.This is a concern for durability of reinforced concrete structures in aggressive environments and should be addressed.