OBJECTIVE To observe the ultrastructural feature and ECP expression in nasal polyps, and aim to explore its role in the pathogenesis of CRSwNP. METHODS 5 CRSwNP, 5 CRS and 5 control patients underwent sinus surgery were gathered to detect its ultrastructure and expression of ECP by in situ hybridization and electron microscopy technique. RESULTS Under electron microscopy, the eosinophilic cells in CRSwNP group increased, its membrane was intact but fold, the feature of pseudopodium, degranulation and cavitation were all found. The expression level of ECP mRNA was up-regulated. CONCLUSIONS The findings suggest that eosinophilic infiltrate and ECP cytologic inflammation reactions may involve in the pathogenesis of CRSwNP.