The ability of a threat system to quickly detect, locate, identify, and accurately track an armored helicopter has a significant influence upon the safety of the target. RCS (radar cross section) reduction is a major consideration in the design of a stealthy armed helicopter. This paper is to provide a systematic method for qualitatively assessing the RCS reduction benefits on the helicopter detection probability. Considering the helicopter radar scatter signal of mirror reflection and edge diffraction at high radar frequency, the RCS computation methods of PO (physical optics) and ILDC (increment length diffraction coefficient) are firstly introduced. Then probability theory is employed to predict the radar detection probability. Moreover, two hypothesis armed helicopters, in which one is conventionally designed and another is stealthily designed with radar absorbent material, are used as an example to illustrate the feasibility of the proposed method. Analysis shows that the benefits of radar signature reduction (i.e., radar absorbent material) include a decrease in the radar detection probability and an increase in the effectiveness of any radar countermeasures.