IL-33 is involved in the pathogenesis of chronic inflammations through the induction of Th2 cytokines and eosinophils. We showed that IL-33 gene expression level was correlated with the blood eosinophils number of patients with pollinosis. It suggests that suppression of IL-33 gene expression could ameliorate IL-33-induced eosinophilic chronic inflammation. It was reported that Brazilian Green Propolis (BGP) has many pharmacological properties. We showed that BGP suppressed the expression of histamine H₁ receptor and IL-9 genes. Here, we sought to investigate the effect of BGP on the suppression of IL-33 gene expression and identify the responsible active compound. The BGP ethanolic extract suppressed PMA-induced IL-33 gene up-regulation in Swiss3T3 cells. It was fractionated using various solvents. The active fraction, then, was further fractionated by silica gel column chromatography, followed by Sephadex LH-20 column. A single compound was detected in TLC, and FTIR and NMR analyses identified the compound as betuletol. Betuletol suppressed PMA-induced IL-33 gene up-regulation. Immunoblot analysis showed that betuletol suppressed PMA-induced ERK phosphorylation. In conclusion, betuletol was identified as an active compound, and it suppresses IL-33 gene expression through the inhibition of ERK phosphorylation.