Abstract： Objective To evaluate the characteristics of exercise ventilation and gas exchange in obese and extremely obese patients by Cardiopulmonary exercise test (CPX).Methods Restrospective analysis of subjects who underwent CPET in Shengjing Hospital of China Medical University from June 2014 to June 2017.A total of 74 subjects were enrolled.According to the body mass index (BMI),74 subjects were divided into control group(18.5 kg/m2≤BMI＜23.9 kg/m2) (n=21),obese group (28.0 kg/m2≤BMI＜40.0 kg/m2) (n=30) and extremely obese group(BMI≥-40.0 kg/m2)(n=23),respectively.VO2max,VO2max/kg,anaerobic threshold(AT),oxygen pulse(VO2/HR),breath reserve(BR),inhale time (VTin),expiratory time(VTex) and ventilatory equivalent for CO2(EqCO2)were measured by CPX and compared by using one-way ANOVA.Results Compared to the control group (1 620±400) I/min,the maximal oxygen uptake(VO2max) in obese group(1 905± 592) L/min and extremely obese group (2 131.09±541.86) L/min were significance higher (F=5.14,P＜0.01).The VO2max/kg in obese group (19±5) L·min-1 ·kg-1 and extremely obese groups (16±4) L·min-1 ·kg-1 were significant lower than those in control group(27±5) L· min-1· kg-1 (F=35.37,P＜0.01).Compared to the control group (9.3±1.4) L·min-1· W-1,the change of oxygen uptake required under certain exercise load (AVO2/AWR) in obese group(9.0± 1.7) L· min-1 · W1and extremely obese group (8.7±2.2) L · min-1 · W-1 were no significant difference (F=0.67,P=0.51).The AT in obese group (1 114±391) L/min and extremely obese group (1 348± 349) L/min were significant higher than those in control group (832±223) L/min (F=12.85,P＜0.01).Compared to the control group(10±4) L· min-1· b-1,Vo2/HR in obese group (12±3) L· min-1· b-1 and extremely obese group(14±3) L· min· b-1 were significance higher (F=8.16,P＜0.01).No significant difference was found between the three groups inBR,VTin,VTex and EqCO2.Conclusion obese and extremely obese individuals have a decreased ablity to exercise when the body requires anaerobic metabolism to provide energy.As exercise power increases,the heart oxygen consumption per stroke and the amount of volume and oxygen required for gax exchange does increase.