In this work, a small amount of CaO single dopant was adopted to realize the densification and microstructure control of fine grained YAG ceramic with excellent optical quality, by a simple solid-state reaction and one-step vacuum sintering method. Then, highly transparent YAG ceramics (T=84.4% at 1064nm) were obtained just after vacuum sintering at 1820 degrees C for 8hours. The average grain size was only 2.7m, when the total amount of CaO was as low as 0.045wt%. The effect of CaO on the microstructural evolution and optical property of the as-fabricated YAG ceramics was systematically investigated in detail. It was found that CaO dopant promoted both densification and grain growth of YAG ceramics when the sintering temperature was lower than 1660 degrees C, however, it dramatically inhibited grain growth when the sintering temperature was further increased.