Abstract: The objective of this study is to divide the types and boundaries of bound water on loess particle surface and to investigate their related physical and mechanical properties using the isothermal adsorption and thermogravimetric analyses. The results indicate that the water formed from the water vapor adsorbed on a loess particle with the relative humidity less than 0.59 is the strong bound water, and the water formed from the water vapor adsorbed on a loess particle with the relative humidity of 0.59 ~ 0.98 is the weak bound water. The absolute water content boundaries of the strong bound water and weak bound water are 0～2.70% and 2.70%～6.61% respectively. The thermo-gravimetric intervals of the strong bound water and weak bound water are 125℃～245℃ and 65℃～125℃ respectively. The hydrous specific gravity of hydrated loess and the density of the bound water decrease with the increase of the absolute water content. The volume of hydrated loess, volume of the bound water and thickness of the combined water film increase with the increase of the absolute water content.