This paper presents a loop-back method for quadrature modulation transceiver circuits to diagnose parametric faults of the system, such as I/Q mismatch, baseband/RF time skew, and DC offsets. The proposed method is capable of separating the transmitter parameters from the receiver parameters in a single measurement. Digital baseband signals are used as test input and the received I/Q baseband signals are analyzed for parameter extraction. Experimental results show that the test method has 1.5\% RMS error performance inthe presence of thermal noise and unknown delays caused by the loop-back connection. The test technique can easily be implemented on a digital tester. Either on-chip baseband data converters or load board data converters (for transceiver circuits without digital output) can be used for interfacing the transceiver circuit under test.