Abstract: Dielectric insulation strength between contact gap and its corresponding variation during travel process are two important performance indexes for the structural design of a high voltage circuit breaker. An experimental circuit was built to investigate the dielectric insulation characteristics between the contacts in a high voltage circuit breaker. The dynamic breakdown voltage between the contacts was measured in C₄F₇N/CO₂ at different pressures and different C₄F₇N contents during no current flows or no arc burns closing and opening process. The closing pre-breakdown and opening insulation characteristics of the C₄F₇N/CO₂ circuit breaker was studied as well. It was firstly found that "low voltage breakdown" appeared in the dynamic breakdown voltage of C₄F₇N/CO₂ contact gap during this process. In the closing process, the dispersion of dynamic breakdown voltage was large, as a result, the closing pre-breakdown characteristics of C₄F₇N/CO₂ circuit breaker were significantly lower than SF₆ circuit breaker. With increasing the contents of C₄F₇N or the pressures in the circuit breaker, no significant improvement could be found in the breakdown voltage dispersion property of the C₄F₇N/CO₂. Moreover, under the positive and negative polarity conditions, the average rate of rise of dielectric strength (RRDS) of 9%C₄F₇N/91%CO₂ mixture at 0.7MPa were 49.8kV/ms and 42.7kV/ms, respectively, with an "reverse polarity" effect. The phenomenon of "low voltage breakdown" mainly concentrated within 2ms (about 6mm contact gap) after the contacts separation during opening process, restrike was easy to occur after the arc extinguishing in this range. Thus, the structural design of C₄F₇N/CO₂ filled circuit breaker should avoid arc extinguishing in this rang.