Made of AISI 4140 steel CT (Compact Tension) specimens subjected toplas manitriding process, temperature of 500 degrees C for 2 hours 50N(2)%-50H(2)% gas mixture. Block Over load (OL) the number of cycles on fatigue crack growth retardation, investigated the effect of plas manitriding. Un nitrided and nitrided specimens, the more the number of cycles of OL increased, the more effect ofthe crack retardation found. The crack retardation by overload cycle numbers 1, 10, 20, 100 and 1000 of, in un nitrided samples effectively occurred, but nitridedat lower levels. Crack growth retardation caused by 3000 the number of cycles of the block overloading, show edhigher levels occurred on the nitrided samples than unnitrided. and the overload cycle in 3000 increased. Structural and mechanical properties of the samples; investigated using x-ray diffraction, micro hardness testing, scanning electron microscopy, optical microscopy and optical digital imaging tools. Digital image analysis method was used to monitor fatigue crack growth.