Using semi-solid isothermal heat-treatment, the behavior of grain coarsening of Al-Zn-Mg-Cu alloy at the temperatures in solid-liquid two phase region was studied. The results from regression analysis show that the relationship between the average diameter of α(Al) grains and the isothermal holding time nearly fits the formula . In the formula, the coefficient of coarsening rate K increases with the increase of the isothermal temperature, which equals to 14811.4 μm4/s at 610 oC and 17836.3 μm4/s at 620 oC, respectively. The microstructure’s polygonizing, spheroidizing and coarsening of the experimental alloy occur simultaneously; among them the coarsening is caused by the coalescence between the adjacent grains and Ostwald ripening. During the early stage of the isothermal treatment, the coarsening is governed by the coalescence growth, but the coalescence becomes more and more difficult with the increase of the liquid fraction, which results in the decrease of the coarsening rate in the late stage