The constant vibration mode and the constant excitation mode in noncontact atomic force microscopy were compared to investigate the force interaction between tip and surface. As a result, we found that the constant excitation mode is much more gentle than the constant vibration mode. We also succeeded in atomic resolution imaging on InP(110) surface not only in the noncontact region but in the contact region for the first time. Furthermore, we found the discontinuity of the force gradient curve on reactive Si(111)7×7 reconstructed surface. We proposed a model to explain the discontinuity with the crossover between the physical and chemical bonding interaction.