This paper describes a low cost, high resolution, built-in test technique for RF mixers which uses a simple RF amplitude detector. The method has been used to predict the performance parameters of a 940 MHz RF mixer. The detector has small area overhead with very low frequency output, which can be sampled at 10MHz. The sampled output waveform is analyzed using an FFT, and the low frequency measurements are used to predict the conversion gain and Third Order Intercept point(TOI) of the mixer. We have completed the design and layout of the 940 MHz test mixer with on chip RF detector in the United Micro Electronics Corporation( UMC) 0.18?m CMOS process. Post layout simulation results show accurate prediction of the mixer specifications.