Objective Observe the differential expression of human beta-defensin-3 (hBD-3) protein in Mycoplasma pneumoniae pneumonia (MPP) patients and compare different hBD-3 protein levels before and after 7-14 days therapy in MPP children to explore the role of defensive mechanisms of innate airway in MPP.Methods Protein concentration of hBD-3 in MPP patients, non-MPP patients and control were detected and compared by ELISA.HBD-3 protein levels before and after 7-14 days therapy in MPP children were also compare.Results Protein concentrations of hBD-3 in MPP were lower than control, while after treatment hBD-3 levels were risen.Conclusion The decreasing of hBD-3 protein was associated with epithelial cell damage, and probably participated in MPP immune system damage.