Abstract: Measurement of shock standoff distances over spheres and the Mars entry vehicle model in CO₂ has been conducted in the hypervelocity ballistic range of Hypervelocity Aerodynamics Institute, China Aerodynamics Research and Development Center. Test models were spheres with the diameter of 10mm and entry vehicle models with the nose radius of 12.5mm. For spheres, the flight velocities were between 2.122 and 4.220km/s with ambient pressures between 2.42 and 12.3kPa. For the entry vehicle model, the flight velocity was 2.802km/s with the ambient pressure of 1.836kPa. Comparison was made between the test data and calculated results using the two-temperature nonequilibrium model. Under present test conditions, the two-temperature nonequilibrium model can basically reproduce the shock standoff distances over the test models. The flow over the test models is speculated to be mainly nonequilibrium. More test data with higher flight velocities (>5km/s) are needed for the validation of the two-temperature nonequilibrium model in CO₂ with higher freestream velocity. The influence of multi-temperature models and different chemical reaction models on the accuracy of the numerical simulation for the nonequlilbirum flow in CO₂ can be further studied.