Abstract: The MLS (Microwave Limb Sounder) satellite observations of carbon monoxide (CO) mixing ratios from 2005 to 2014 have been analyzed to study the inter-annual variation of carbon monoxide (CO) in the tropical stratosphere. The results show that the quasi-biennial oscillation (QBO) in the tropical stratospheric CO mixing ratios has a phase change at 30 hPa. The results from a chemistry-climate model show that the CO QBO is primarily resulted from combined effects of dynamical and chemical processes associated with CO, while the dynamical process is mainly associated with the transport of CO by the QBO-induced meridional circulation. The combined effects of dynamical and chemical processes lead to a reverse of the CO vertical gradient in the tropical stratosphere at 30 hPa. Consequently, the CO QBO exhibits a phase change around this level. The model results also show that the chemical processes associated with CO not only have a weakening effect on the amplitude of the CO QBO, but also lead to a QBO phase difference by 3 months between zonal wind QBO and CO QBO at 10-30 hPa.