This study aims to develop an in situ observation technique of micron-scale metal specimens under fully-reversed tension-compression cyclic loading and to investigate the unique cracking process in low-cycle fatigue. The fatigue experiment was conducted on a single crystal copper micron-scale specimen under a constant displacement amplitude. While crystallographic slip spread over the entire test section during the tensile first half-cycle, a locally concentrated slip band appeared during reverse loading (compression). In situ observations using scanning electron microscopy revealed that crystallographic slip occurred only near the localized band in consecutive cycles, and strain localization due to slip bands led to nanoscale extrusion/intrusion at the surface. This indicates that, unlike during the fatigue of a typical bulk pure metal, no characteristic dislocation substructure was formed during the extrusion/intrusion process. The extrusion/intrusion was the cause of failure of the micron-scale metal specimen in fatigue. Moreover, the process required a much higher stress amplitude than in the case of bulk copper specimen. Thus, the fatigue cracking process for copper micro-scale specimen differs significantly from that for bulk copper specimen.