The invention relates to an upper panel assembly and a circuit breaker that uses the upper panel assembly. The circuit breaker comprises a circuit breaker cart and the upper panel assembly. The upper panel assembly comprises an upper panel and a stiffener, wherein thickness of the upper panel extends in a front-rear direction. The stiffener comprises an upper panel connecting section for connecting to the upper panel and a cart connecting section for connecting to the circuit breaker cart, wherein the upper panel connecting section and the cart connecting section are disposed one above the other. The upper panel connecting section is located on a front side of the upper panel, and the cart connecting section is located on a rear side of the upper panel. As only the cart connecting section on the stiffener is located on the rear side of the upper panel, the stiffener has a relatively small effect on the insulating property of the circuit breaker.