Abstract: Objective: To explore the application in clinical diagnosis of mycoplasma pneumonia antibody titer.Method: Using gelatin particles agglutination test for serum antibody detection,the titer of paired serum antibody with the interval of two weeks which had a 4 times increase or decrease at least was used to confirm the diagnosis of mycoplasma pneumonia infection.Result: Among 251 cases of mycoplasma pneumoniae antibody-positive patients with serum antibody titers of 1:40 or 1:80 positive patients with an interval of 2 weeks,there were 8 cases in which antibody titers were more than four times increased,and 110 cases in which antibody titers were less than four times increased. Among serum antibody titers higher than or equal to 1:160 positive patients with an interval of 2 weeks,there were 130 cases in which antibody titers were more than four times increased,and 3 cases in which antibody titers were less than four times increased or even decreased.Conclusion: The serum antibody titers less than 1:160 indicated early infections or past infections,and mostly past infections. The antibody titers higher than 1:160 indicated new infections with a high possibility. The higher the antibody titers,the higher the possibility of new infections.