Built-In-Test (BIT) for Radio Frequency (RF) integrated circuits can reduce the testing cost, especially with the increase of integration level and operating frequency. A fully integrated CMOS BIT detection circuit is presented in this work. This BIT detection circuit is rectifier-based and low threshold voltage diode-connected MOS transistor with substrate positively-biased is used to improve the detecting sensitivity. As an example, a 2.4GHz LNA is used, the high frequency small signal gain is extracted and the gain fluctuation due to Process, supply Voltage and Temperature (PVT) variations is also investigated. The simulation results show that this BIT detection circuit can realize on-chip functional verification of RF circuits and also monitor the influence of PVT variations on the performance of the circuit without affecting the high frequency performance of the measured RF circuits.