Corresponding to the expanding demand of the power transmission and distribution system in USA, a new type of 800kV 50kA gas circuit breaker (GCB) with double-break and single mechanism was developed based on the current one with double mechanisms, matching the newest standard, and improving the reliability. All type tests, including breaking tests, mechanical tests and temperature rise test etc., were successfully completed. In the present paper, ratings, features, type tests and products of the GCB are introduced.