Abstract: The effect of hapten space arm length on immune recognition by determination of the titers and affinities between three coating antigens and antibodies of MCPA and quinclorac were discussed, respectively. As a result,the titer of the antibodies induced by the haptens with four carbon chain are the highest; It indicated that the same antibody is apt to recognize the hapten with longer space arm; To the same antigen,the recognition capability of the antibody for MCPA induced by the hapten with none space arm is the strongest,while that for quinclorac induced by four cabon atom is the strongest,which indicated that the recognition capability of different antibodies is concerned with the space configuration of IgG.