To observe propagation characteristics of microstrip lines on YIG film, the FDTD is employed. For various GGG substrate and YIG film, the dispersion curves and the bandwidth of the cutoff region are calculated. And the distribution of the electric fields of the transverse plane underneath the conductor strip is obtained near upper and lower cutoff frequency. The results of two and three dimensional FDTD for the upper cutoff frequency are compared with spectral domain approach(SDA). A good agreement is verified, although there is relative error 6% between the results calculated by the two methods.