Carbon fiber reinforced plastic (CFRP) composites were widely used in machinery, automobile, and aerospace industries due to the superior properties of high stiffness, high strength-toweight ratio, high fatigue resistance, and good wear resistance. In these applications, hole making on CFRP composites was needed for assembly purpose. In different hole making processes, delamination was considered as a crucial issue, leading to premature failure of products. Compared with the conventional CFRP hole making processes (such as twist drilling, milling, grinding, etc.), rotary ultrasonic machine (RUM) had advantages of low cutting force and torque, low surface roughness, and long tool life. Therefore, numbers of researchers worked on RUM processes. The reported investigations mainly focused on the effects of input variables (including machining variables, tool variables, and workpiece properties) on cutting force, torque, surface roughness, and tool life, etc. In addition, effects of cutting tool variables on delamination were also reported. However, there were no reported investigations on effects of machining variables on delamination in RUM of CFRP composites. This investigation, for the first time, reported the effects of ultrasonic power, tool rotation speed, and feedrate on delamination as well as its associated cutting force. Based on the experiments, the results showed that delamination decreased with ultrasonic power increasing, tool rotation speed increasing, or feedrate decreasing. The relationships between cutting force and delamination were also studied.