Abstract: In this paper, the particles deformation and combination of the biomass materials in the compressing and forming process was studied. By compressing the biomass materials in the cylinder, and measuring the two dimension diameter of the particles of the formed materials with the right microscope, it is show that particle is expanded in the vertical direction with the maximum main stress, and the two dimension diameter is linearly increase with the compression pressure. The relation of the two dimension diameter of the particles and the temperature of the cylinder, the water content of the materials, particle size, the kind of the materials and etc. was also analysised. In view of the present result, the particles combination model of the biomass materials compressing in the cylinder was proposed.