A planar waveguide model is presented for calculating dispersion characteristics with the frequency dependent effective dielectric constant in microstrip lines and results are compared by the variation of each parameter. It is compared to use a wide range of relative dielectric constants and the strip $h_{width}$strate height, W/h ratios, 0.9$leq$W/h$leq$2. As the result of a computer simulation, the normalized phase velocity using a planar waveguide model for each case is more closely approached to 1/$sqrt{epsilon_r}$ as the increasing of the frequency than the other method that has already been presented.