Abstract: Numerical simulation is conducted to investigate two-dimensional frictionless disks under isotropic compression.Results with different simulation conditions are contrasted.It shows that monodisperse system is easier to crystallize.Its critical packing fraction φ_C and critical mean contact number Z_C are larger than bidisperse system.On J point,relations of packing fraction increment Δφ to mean contact number increment ΔZ and average overlap δ are almost irrelevant to simulation conditions.Relation of pressure P and packing fraction increment Δφ is only determined by interaction form.These relationships can be fitted very well by power functions.