Carbon Fiber Reinforced Plastic (CFRP) is a typically hard machining material. The changing relationship between the milling speed and feeding rate along with milling force was researched during milling forming slots of CFRP, by using the forming tool of carbon carbide with the different technological parameters in machining. The machining fault on exit surface of milling and its changing relationship in regard with milling force were analyzed. The experimental results have shown that the milling force is lowered with a higher milling speed, and is reduced with increasing of feeding rate. The fault of the exit surface of milling is mainly delamination, burr and non-cutting off of materials, and the area of delamination is increased with getting higher of milling force.