【Objective】 To explore the change of C-reactive protein(CRP) and the occurrence of extra pulmonary organ injury in the children with different titers of mycoplasma pneumoniae (MP) -IgM after MP infection. 【Methods】 300 cases with MP infection were divided into 3 groups according to the titers of MP-IgM,group A with titer of 1∶80,group B with titer of 1∶160 and group C with titer equal to or above 1∶320.The positive rate of CRP and the occurrence of extra pulmonary organ injury were retrospectively analysed in the different groups. 【Result】 Higher positive rate of CRP and also higher occurrence of extra pulmonary organ injury were observed in the group C compared with group A and B. 【Conclusion】 The determinations of CRP and the titer of MP-IgM are useful for early diagnosis,observation of the disease course and prediction of the prognosis of children with MP.