A light efficiency enhancing optical device is disclosed, including a cholesteric liquid crystal film, a quarter wave plate disposed on a light out-going surface of the cholesteric liquid crystal film and an optical compensating film disposed on a light out-going surface of the quarter wave plate, wherein the optical compensating film includes a positive birefringence C-plate, and a composite optical compensating film with combination of the optical compensating film and the quarter wave plate has in-plane phase retardation R0 of 100 nm˜170 nm and out-of-plane phase retardation Rth of 0 nm˜400 nm.