Ni-based catalysts supported on various alumina supports with different crystalline phases (gamma-, alpha-, theta- and delta-Al2O3) were prepared and the effects of crystalline phases on the catalytic performance towards acetic acid steam reforming (AASR) were investigated. An acetic acid conversion of nearly 100% was observed in all the four catalysts, and their hydrogen selectivities were in the following order: Ni/alpha-Al2O3 (90%) > Ni/gamma-Al2O3 (79%) > Ni/gamma-Al2O3 (53%) > Ni/theta-Al2O3 (25%). Using different characterization methods, the inner relationship between catalyst crystalline phase and catalytic properties was determined. Through TEM, H-2-TPR and XPS characterization, Compared with alpha-Al2O3,a on the surface of other crystalline phases of Al2O3 support were formed NiAl2O4 which indicated stronger interaction intensity between these supports and Ni., and that would reduce the formation of metallic Ni. It was confirmed that metallic Ni played a core role of catalytic AASR. More metallic Ni content caused better C-C bonds and C-H bonds breaking capability and eventually enhanced the selectivity towards hydrogen. That would be the key reason for Ni/alpha-Al2O3 showed best hydrogen selectivity among these four catalysts. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.