The interaction between single synthesized antigen and its monoclonal antibody was directly monitored by Atomic Force Microscopy (AFM) with antigen-labeled AFM tip. The details about tip modification and antibody immobilization were described. The interactions were monitored by the change of adhesive force in separating the bound antigen-antibody. The results showed that two processes could be observed in the combination of antigen-antibody system. Possible mechanisms of rupture process as independent unbinding and cooperative unbinding were discussed. At slow separation process, the antigen-antibody linkage appeared to rupture sequentially. Increasing the separation rate from 50 nm/s to 2000 nm/s led to an increase in the possibility of cooperative unbinding. The pH influenced the affinity constants greatly, but had little effect on the magnitude of unbinding forces. The obtained results were compared with affinity constants between complete antigen and antibody obtained by immunoassay method (ELISA). The observed single rupture force between individual antibody and antigen molecules provided pertinent information relating to the manner how the antibody molecule binds to its specific antigen.