A novel heterogeneous catalyst, the ionic liquid (IL) of 1-butyl-3-methylimidazolium acetate (BmimOAc) immobilized on MIL-101-NH2, denoted as IL(OAc-)-MIL-101-NH2, was prepared by the "ship-in-a-bottle" strategy. The IL of BmimOAc was prepared in the MIL-101-NH2 nanocages primordially, in which the condensation product of MIL-101-NH2's amine group with 1,1'-carbonyldiimidazole (CDI) reacted with 1-bromo butane, and then the intermediate exchanged with potassium acetate. The structure and physicochemical properties of IL(OAc-)-MIL-101-NH2 were characterized by powder X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, DRS UV-vis, nitrogen adsorption-desorption, and elemental analysis. The results indicated that BmimOAc was anchored in the MIL-101-NH2 skeleton via the acylamino group and confined in the nanocages in the form of a single molecule. The composite material of IL(OAc-)-MIL-101-NH2 exhibited excellent catalytic activity and catalytically synthesized 3-aryl-2-oxazolone in an excellent yield of 92%. It can be reused up to six times without noteworthy loss of its activity and demonstrated distinct size-selective property for substrates. It was conjectured that the diffusion kinetics of reactants could be controlled by the aperture size of the metal-organic framework support.