Objective To establish the assay method of hydroxyproline(HYP) level in rat lung tissue for evaluating the lung fi-brosis degree .Methods The impact of different acid hydrolyzable time ,oxidative time ,developing time on the assay results of the HYP level in rat lung tissue was studied .On this basis ,the assay method for determining the HYP level was established and prelim-inarily applied in the detection of the HYP level in rat lung tissue .Results The optimal condition for the HYP level detection in rat lung tissue was as follows :7 .50 mol HCL was hydrolyzed for 16 h at 110 ℃ ,oxidized for 10 min at room temperature and devel-oped for 25min at 60℃ .The sensitivity of this method was 0 .067μg/mL .The recovery rate and the average CV of this method were 88 .85% -110 .88% and 4 .70% -6 .60% ,respectively .In the study of bleomycin induced rat lung fibrosis ,the HYP level of the model group was obviously higher than that of the control group .Conclusion This method has high sensitivity ,high recovery rate and good reproducibility ,and may be used as a reliable quantitative method to judge the lung fibrosis level in clinic .