Surface machining of carbon fiber reinforced plastic (CFRP) using rotary ultrasonic machining (RUM) with vertical ultrasonic vibration was effective in reducing many issues, including high cutting force, high torque, and high tool wear rate. The vertical ultrasonic vibration also induced damages to machined CFRP surfaces and then resulted in increased surface roughness. To simultaneously decrease surface roughness and cutting force, the direction of ultrasonic vibration needed to be parallel with the surface generation direction (horizontal feeding direction). The horizontal ultrasonic vibration was then developed and applied for RUM surface machining of CFRP. The application of horizontal ultrasonic vibration in RUM surface machining produced simultaneously decreased surface roughness and cutting force. However, there were no investigations on delamination in such a process, and delamination was considered as one of the major factors to reject the machined CFRP products. This investigation would study the delamination under different machining-variable groups, the delamination generation mechanisms, and the relationships between delamination and cutting forces through the experimental method in surface machining of CFRP using RUM with horizontal ultrasonic vibration. Smaller cutting force and delamination thickness would be produced by the smaller depth of cut, smaller feedrate, or larger tool rotation speed. Smaller indentation depth was generated by larger tool rotation speed or smaller feedrate. Smaller material removal rate and abrasive-grain number taking part in the cutting process were produced by the smaller depth of cut. The delamination initiation at larger uncut CFRP thickness would be induced by higher cutting force.