We studied the crystallization mechanism of ultra-fast phase change optical disks with recording layers made of GaSb material for digital versatile disk (DVD) systems. The results of a static recording test and an amorphous mark formation simulation suggest that GaSb maintains a high crystal growth rate even at temperatures 150 degrees lower than the material's melting point. Disks with recording layers made of this material have a write speed margin ranging from DVD 3× to 8× or more.