In concrete, it is well known that around the tip of a major crack, a fracture process zone (FPZ) exists wherein toughening mechanisms such as crack nucleation, micro-cracking and aggregate bridging occur. These processes in FPZ are responsible for its post-peak softening behaviour under monotonic loading. Behaviour of concrete under fatigue is more complex and it is seen that concrete under ... [Show full abstract] fatigue loading fails suddenly without any warning raising questions about the formation of FPZ. In the present work, experiments are performed on notched plain concrete beam specimens under three-point bending, using three different sizes of geometrically similar specimens to explore the formation of the FPZ under fatigue loading. The non-destructive tools such as acoustic emission technique and digital image correlation (DIC) are used to study the evolution of internal micro cracks and surface deformation respectively. Experimental results have shown that FPZ formed under fatigue consist only of isolated and distributed micro cracks. The redistribution of stresses takes place only at the final failure cycle leading to unstable macro crack propagation and sudden failure whereas in the monotonic case, the redistribution of stresses takes place because of toughening mechanisms leading to gradual increase in the macro crack.