Low temperature growth Pr0.7Ca0.3MnO3 (PCMO) thin film showed high performance in electric field induced resistance switching (RS). To understand the micro-mechanism of RS in Metal/PCMO/Metal devices, structure evolution of PCMO under external electric field monitored inside transmission electron microscope (TEM) were performed. Evolution of the modulation stripe in as-grown PCMO sample was investigated when applying electric field. The new-generated modulation stripe gradually disappeared. These results indicate that oxygen ion migration plays a key role in RS.