The temperature dependence of the free volume in SMMA/SMA (30/70) blend was investigated by the positron annihilation life spectroscopy (PALS) at different heating rates according to the free volume theory, and the thermodynamics characteristics of the phase behavior of the polymer blends was also explored. At temperatures above Tg, we found from the positron annihilation life spectra that when the structural relaxation time is comparable with the retention time, the temperature dependence of the free volume parameters obviously departs from the linear relation. Based upon the change tendency of the free volume concentration I3 with temperature, it is postulated that there is a growing stage of the phase separation in the same temperature range.