Mg-Cu-Y metallic glass thin films were deposited by magnetron sputtering. Mg58Cu29Y13 metallic glass thin film has a large supercooled liquid region (Delta T-X = 57.0 K) and a smooth surface (R-a = 0.485 nm). The phase separation and nanocrystallization properties of it were investigated. Scanning electron microscope observation and energy disperse spectrum analysis in the single line scan transmission electron microscope test show phase separation in the Mg58Cu29Y13 metallic glass thin film annealed slightly above glass transition temperature for 1 min. Transmission electron microscope studies show 10 nm diameter nanocrystals in the fully crystallized specimen. The crystallization behavior of it is very different from that of bulk metallic glasses. The nanocrystals structure in the film above crystallization temperature is attributed to the high nucleation rate and low nucleus growth rate in the crystallization process. (C) 2013 Elsevier B. V. All rights reserved.