A new 8 mm high-performance sum-frequency upconverter is described. Ttsconversion loss is 2.9 dB; its output power is 1.34mW at 1 dB compression point. As the power of sum-frequency signal below 1mW, the upconverter shows a very good linearity. Sum-frequency signal is 30 dB higher than stray spectrum. The isolation of each port is over 40 dB.