Alumina based eutectic in situ composite is now considered to be a promising candidate for ultrahigh temperature structural materials due to its excellent performance even close to its melting point. In this work, Laser Floating Zone (LFZ) solidification experiments have been performed on Al2O3/Y2O3 containing 18.5 mol% Y2O3 eutectic in situ composite at growth rate between 8 and 800 mu m/s. The solid/liquid (S/L) interface morphology of this system was reported. Based on the interface morphology, the lamellar selection mechanism of irregular eutectic systems was also discussed. The solidification microstructure evolution was studied, and the characteristic scale of irregular eutectic systems was analyzed using a mathematical model considering non-isothermal interface morphology. An important parameter, Phi, which represents relationship between characteristic scales, was also given. (c) 2012 Elsevier B.V. All rights reserved.