Objective: To investigate the value of indirect immunofluorescence (IFA) and passive agglutination (PPA) for the diagnosis of Mycoplasma pneumonia (MP) infection in children with lower respiratory tract infection. Methods: The detection of serum MP-IgM via PPA and IFA in 388 children with lower respiratory tract infection was analyzed, including the difference in positive rates of the two methods in different time points during the course of disease. Results: The overall positive rate of MP-IgM was 32.5% (126/388). The detecting results were influenced by days of fever, and there was no significant difference between the two methods when the fever was 0-3 days (P>0.05). There was a difference in positive detection rates between the two methods with the increase in days of fever. For fever more than 4 days, the positive detection rate of PPA (25.14%, 119/388) was higher than that of IFA (18.0%; 70/388) (P<0.05). A lack of consistency was existed between the two methods (Kappa=0.126). The specificity of IFA method (97%) was higher than that of PPA (82%), and the sensitivity of PPA method (90%) was higher than that of IFA (53%). Conclusions: In children with mycoplasma pneumonia infection of lower respiratory tract, there is no significant difference in positive detection rate between the two methods for cases with no fever or with fever 1-3 d. However, for cases with fever over 4 days, detection of MP-IgM using combined PPA and IFA is conducive to the positive detection of MP-IgM.