We report a method to measure the net loss and gain of a terahertz quantum cascade laser (THz-QCL), which is based on a master-oscillator power-amplifier (MOPA) configuration. In the measurement, the master-oscillator (MO) section and power-amplifier (PA) section are separately biased. With the fixed seed power from the MO section, the output power of the device is measured at various bias voltage applied on the PA section, from which the net loss and gain is deduced as a function of bias. We demonstrate that gain clamping and gain saturation can be avoided in our measurement, which allow a complete evolution of the loss and gain characteristics. For a THz-QCL with the bound-to-continuum active region and the metal-metal waveguide, the measured maximal net gain is about 13.0-16.5 cm(-1) at 2.58 THz at 20 K, which is in qualitative agreement with the theoretical analysis.