Contact between tip of atomic force microscope(AFM) and tested sample surface is simulated using molecular dynamics (MD).Relationship between contact force and gap is analyzed.Surface pressure distribution is discussed.Phenomena of "jump-to-contact" and "neck-separation" are observed.Hysteresis between contact process and separate process is obtained.Relationship of surface distortion radius with contact force and gap is also discussed.Only atomic layer close to contact area affects the contact force.Surface atoms close to the contact are pressed and those far from the contact are pulled.