A composition for prevention or treatment of pulmonary fibrosis comprising chromenone derivatives represented by Formula I of the present invention or pharmaceutically acceptable salts thereof as an active ingredient, produces altered heat shock protein 27 (HSP27) dimers, thereby preventing normal HSP27 non-phosphorylated polymers from performing a chaperone function and reducing a cell protection function that chaperone originally performs. Thus the inventive composition has a remarkable inhibitory effect on pulmonary fibrosis, in particular, an irradiation-induced pulmonary fibrosis phenomenon.