Abstract: In this paper, fatigue tests were carried out for different type and size of specimens to study fatigue damage and crack initiation of LY12CZ plate. Using microscopy observation and strain analysis, we have studied tension-tension low cycle fatigue damage. The experimental results show that small crack initiation damage zones exist in the regions of high strain. The size of damage zone is a characteristic value related to material and specimen geometry. Some small cracks in th damage zone merge into a macro-crack and others become non-propagation small cracks. The fatigue damage evolution law describing damage change along with fatigue cycles has been obtained.