The temperature dependences of Faraday rotation spectra (FRS) at around 1.5 μm are measured at temperatures between 60 and 300 K for a single crystalline Co 2 + -substituted magnetic garnet film grown by a liquid-phase epitaxy method onto a gadolinium-gallium garnet substrate. FRS depend strongly on the temperature, and increase with a decrease of temperature with fine structures. FRS at some temperatures are calculated by a molecular field approximation, and the overall features of FRS are well reproduced by assuming the molecular field of 80<S F e z > cm - 1 , where <S F e z > is a thermal average of an iron spin S z in magnetic garnets.