Abstract: By controlling the initial grain size of powders and sintering conditions of the green body, Y-TZP/Al2O3 nano composite ceramics with grain size of 100-500 nm were prepared. The microstructure and phase composition were analyzed by means of scanning electron microscopy and X-ray diffraction. Using indentation method the micro-hardness of the ceramics with different grain sizes was evaluated. The results show that the ceramics were composed of t-ZrO2 with little α-Al2O3 phase. The micro-hardness of the ceramics had obvious indentation size effect, and the hardness was inversely proportional to the square root of grain size.