The effect of processing parameters, including deformation temperature, strain rate and deformation degree on the microstructure evolution and the microstructure variables (grain size and volume fraction of primary α phase) was investigated based on the microstructure observation and the quantitative metallography of the isothermally compressed Ti-6Al-4V alloy. The results show that the curves of grain size of primary α phase are oscillatory in the α+β two-phase region with increasing of the deformation temperature, and meanwhile the volume fraction of primary α phase decreases. The effect of the strain rate on the morphology of primary α phase is significant, while its effect on the microstructure variables of isothermally compressed Ti-6Al-4V alloy is dependent on deformation temperature. The grain size of primary α phase decreases with increasing of strain rate above 1203 K, but below 1203 K it fluctuates. The curves of volume fraction of primary α phase are oscillatory with increasing of strain rate above 1223 K, but below 1223 K the volume fraction decreases. The grain size of primary α phase increases slightly after a drop with increasing of the deformation degree, and meanwhile the secondary α phase decreases gradually. The effect of deformation degree on the volume fraction of primary α phase is not significant