An experimental set-up consisting of a measuring device of magnetooptic rotation and a quarter-wave plate is used to measure the magnetic circular dichroism (ellipticity tanh θ") of (BiTm)3(FeGa)5O12 single crystal films and the change of θ" as a function of the magnetic field intensity H and sample thickness 2h. The measuring accuracy is 0.01°. Experimental results have proved that the imaginary part of magneto-optie rotation is also related to the magnetization intensity M just as the real part, and that it changes linearly with the sample thickness.