This paper reports on a high output performance of a Ka-band monolithic HEMT Voltage Controlled Oscillator(VCO). This VCO has a sub-resonator in order to avoid reduction in Q-factor of a resonator. Circuit elements of the sub-resonator are optimized to achieve a wide tuning range as well as high output power and low phase noise performances. In addition, an AlGaAs/InGaAs double-hetero structure High Electron Mobility Transistor(HEMT) is employed in the VCO to obtain a high output performance. A high output power of 19.4 dBm has been achieved at an oscillation frequency of 36.2 GHz. This performance has been achieved without any buffer amplifiers. A tuning range of more than 2.5 GHz is also obtained with a stable high output power. To our knowledge, this represents the highest output power of monolithic VCO without any buffer amplifiers.