Abstract： Objective To investigate the value of the parameters of the within-breath behaviour in chronic obstructive pulmonary disease (COPD) measured by impulsive oscillation system (IOS),evaluate the differences in parameters during tidal breathing,and assess the relativity of the parameters between IOS and pulmonary function test.Methods Mild (n =4) moderate (n =15),severe (n =18),very severe (n =10) COPD patient and normal (n =6) were recruited to receive IOS and PET at Beijing Friendship Hospital.The parameters of IOS was assessed using MostGraph-01 (Chest MI,Inc,Tokyo,Japan),MostGraph-01 could depict the parameters in a frequency-and-respiratory-cycle-dependent manner in 3-D graphics.Results Compared with controls,R5,R5-R20,Fres in COPD groups increased significantly (P ＜ 0.05),X5 in COPD groups decreased significantly (P ＜ 0.05).In inspiratoryexpiratory breath analysis,R5,R5-R20,X5,Fres changed dynamically during tidal breathing in COPD,R5,R5-R20,Fres in expiratory phase increased and X5 fell to a more negative level in a severity-dependent fashion.R5,R5-R20,X5,Fres in expiratory phase were positively correlated with disease severity.R5,R5-R20 were negatively correlated with FEV1,FEV1/FVC,FEV1 ％ pred (P ＜ 0.05),Fres was negatively correlated with FVC,FEV1,FEV1/FVC,FEV1 ％ pred (P ＜0.05),X5 was positively correlated with FEV1,FEV1/FVC,FEV1 ％ pred (P ＜0.05).Conclusions IOS can assess the within-breath behavior of the oscillatory mechanics,which could be serviceable to evaluating the severity of COPD.They changed dynamically during tidal breathing,the parameters in expiratory phase are suitable for appraising of COPD.