In recent years

CO

2

gas as a potential alternative to SF

6

attracts much attention in the power industry.The electrical breakdown characteristic is the key indicator for its performance evaluation.In this paper

the mathe-matical expression for calculating breakdown voltage and critical electron temperature is derived on the basis of gasstreamer discharge criteria

the des

cription of relationship between macroscopic variables of electrical breakdown（the ratio between breakdown voltage

breakdown electrical field and pressure）and the fundamental microscopicvariable（critical electron temperature）of CO

2

and O

2

gas mixture is set up

and the electrical breakdown perfor-mance of the gas mixture at different pressure and electrode gap is studied from the point of view of micro level.It isshown by the result that with the increase of product value of pressure and electrode gap

the critical electron tem-peratures reduces quickly at first and then approaches to constant values.The critical electron temperature of CO

2

-O

2

gas mixture reduces significantly with the increase of contents of CO

2

and shows remarkable synergetic effect.The critical electron temperature of pure CO

2

is about 2.34 eV and is 66.1%of that of pure O

2

（3.54 eV）.Adding acertain proportion of O

2

to CO

2

can improve effectively its electrical breakdown characteristic.In addition

the validi-ty of calculation method and parameter selection in the paper is verified by way of comparing the calculation valueand the literature experimental value of breakdown electric field and pressure ratio of CO

2

-O

2

gas mixtures.