The two-dimensional (2D) crystallization of C-60, driven by the anion adsorption, at the [bmim][PF6]/Au(111) interface has been investigated by in situ scanning tunneling microscopy (STM). It is found that although a number of small ordered domains of C-60 exist in air, they will gradually merge into some big ones in [bmim][PF6]. The formation of a high-quality epitaxial C-60 film is attributed to the slow replacement of C-60 by the PF6 adsorption and the high mobility of C-60 on PF6/Au(111), i.e., a competitive adsorption- desorption process between C-60 and PF6. Such strategy could be used to prepare epitaxial films of functional molecules.