Abstract: To investigate the nonequilibrium flow characteristics under Mars entry condition, shock standoff distances over free flight spheres with flight velocities from 5 to 7 km/s in CO₂ are measured in the ballistic range at Hypervelocity Aerodynamics Institute of China Aerodynamics Research and Development Center (HAI, CARDC). Test data are measured by the shadowgraph and compared with calculated results, based on which the temperature and species profiles of the test flow field are further calculated. Shock standoff distance is generally supposed to decrease monotonously as the free steam velocity increases. However, it is found through the present test results that, the shock standoff distances over spheres actually increase with the increase of the flight velocity from 5.5 to 7.0 km/s. Using Park's two-temperature model and a 5-species 6-reactions chemical reaction model can basically reproduce the measured shock standoff distances of the present test. It is shown from the test results that the flow field shortly after the shock over the spheres is mainly in theromchemical nonequilibrium. The specie CO starts to dissociate at the free stream velocity from 5.5 to 7.0 km/s, which is the possible cause of the increase of the shock standoff distances over spheres within this range of velocity.