Al₂O₃-YAG eutectic ceramic has become an ideal alternative to high-temperature alloys because of its excellent high temperature strength, high oxidation resistance and high temperature structural stability. The technology of laser engineered net shaping was used to prepare the Al₂O₃-YAG eutectic ceramic thin-wall sam-ples. These samples were prepared on a common substrate and a water-cooled constant temperature substrate respectively. Their microstructure and microhardness were compared. The results show that the microstructure of thin-wall sample prepared on the common substrate is three-dimensional network structure with an average eu-tectic spacing of 0.96 μm. And the microstructure of top part of the sample prepared on the water-cooled constant temperature substrate is colony structure, while the microstructure of bottom part is dendrite structure which grows in the reverse direction of the heat flow. The average eutectic spacing of samples prepared on the wa-ter-cooled substrate has reduced to 0.21 μm. Compared with the microhardness of two kinds of thin-wall sam-ples prepared on the different substrates, it is found that the microhardness of the thin–wall samples prepared on the water-cooled constant temperature substrate is increased by about 10%.