Abstract: To analyze the influence mechanism of diamond grain size change on the grinding quality of diamond material, a molecular dynamics method was used to establish the grinding model of spherical rigid diamond grain grinding diamond workpiece, and the grinding force variation law, the stress and the phase transformation distribution under different abrasive grain radii were studied. The results show that when the abrasive grain radius increases from 6a to 20a (a is the lattice constant of diamond), the average normal force and the average tangential force increase linearly, but the increment of the average normal force is 3 times of that of the average tangential force. The shear effect between the abrasive grain and the workpiece is smaller and compared extrusion and friction, and the fluctuation amplitude of the grinding force becomes larger, which indicates that the dislocation formation of workpiece is more severe. At the same time, the strong compressive stress zone formed under the abrasive grain expands and so do the concentrated tensile stress zone caused by friction in the area behind the abrasive grain and the amorphous transformation zone.The number of the defects behind the grinding zone increases, and the machined surface quality of the workpiece becomes worse. When the indentation depth of the abrasive grains with radii of 10a, 15a and 20a is 2 nm, the microhardness of the workpiece surface after abrasive grains scribing is 2.8%, 9.6% and 18.3% lower than that without abrasive grain scribing, respectively. The increase of the abrasive radius will significantly reduce the surface mechanical properties of the workpiece.