The rotational spectra of two isotopologues have been recorded and interpreted for the van der Waals complex chlorotrifluoroethylene-argon, by using pulsed jet Fourier transform microwave technique. Argon locates out of the ethylene plane, in the FCCF cavity of chlorotrifluoroethylene. The preference of the CFCl side over CF2 can be attributed to the presence of "π-hole" over the double bond in chlorotrifluoroethylene. Non-covalent interaction analysis indicated that the van der Waals interaction occurs between argon and two sites of chlorotrifluoroethylene (the π-system and the F atom of the CFCl top). The steric effect between Ar and the C-Cl bond repulses the Ar atom toward the C-F side.