Objective To explore the clinical value of measuring angle alpha ( α) in the descending limb of maximum expiratory flow-volumem ( MEFV) curve in chronic obstructive pulmonary disease ( COPD) and bronchial asthma. Methods From MEFV curves, angle αand other parameters were measured in 157 individuals with COPD and 149 patients with asthma who received pulmonary function test in Zhongshan Hospital, from November 2008 until December 2009. The results were grouped and analyzed.The area under ROC curve of the angle αwas measured in the patients with COPD and compared with asthma. Results The angle αlessened with the increasing degree of airway obstruction in the patients with COPD and asthma( P lt;0. 01, P gt;0. 05) . Tangent( 180 - α) and FEV1. 0 were correlated either in the COPD group or in the asthma group( r = - 0. 640, P =0. 000; r = - 0. 206, P = 0. 012) . There was no statistical difference in the angle α between the COPD group and the asthma group when the patients had mild obstructive ventilation disorders( P =0. 177) . The angle αin the COPD group was smaller than that in the asthma group when the patients had moderate and severe obstructive ventilation disorders( P =0. 000, P =0. 000) . Area under ROC curves of the angle αin the patients with COPD who had mild, moderate and severe obstructive ventilation disorders were 0. 431, 0. 846 and 0. 928 respectively. In moderate obstructive ventilation disorders, the optimal diagnostic cutoff point was 135. 5 degree which was capable of differentiating COPD and asthma with a sensitivity of 81. 7% , a specificity of 74. 2% , a positive predictivevalue of 75. 4% , and a negative predictive value of 80. 7% . In severe obstructive ventilation disorders, the optimal diagnostic cutoff point was 129. 5 degree with a sensitivity of 87. 5% , a specificity of 84. 0% , a positive predictive value of 94. 3% , and a negative predictive value of 77. 8% . Conclusions Angle αin the descending limb of MEFV curve can assess the degree of airway obstruction in COPD, whereas it just reflect the presence of airway obstruction in asthma. Differences in Angle αare associated with pathological and pathophysiological differences between COPD and asthma, which will help for deffirentiating diagnosis.