The cooling dielectric spectra of the -relaxation in dimethyl phthalate, diethyl phthalate, dibutyl phthalate, and dioctyl phthalate series materials are measured, and the average relaxation time of the -relaxation a as a function of temperature T is obtained. By fitting the a data to the empirical Vogel-Fulcher-Tammann law a = 0 exp (A/(T-T0)), the values of 0, A and T0 of the series materials are obtained. The results of 0, A, T0 and Tg show some variation regularities with the carbon number n in the side-group of dimethyl phthalate series molecules. And specifically, with the increase of n, the internal degrees of freedoms of the molecules, A and Tg indicate almost the same tendencies, i.e. first coming down and then going up, while 1/0 and T0 have quite similar behaviors, i.e. first reducing rapidly and then keeping at near constant values.