In this article, a novel concept of 3D integrated V-band diplexer, which permits the realization of compact size using a dual-mode cavity and four single-mode cavities, has been realized in low-temperature cofired ceramic technologies. The dual-mode cavity resonator acting as one resonator for both Rx and Tx filters is developed to generate two resonant modes (TE 102 and TE 103 ) at the center frequency of Rx (56.5-58.5 GHz) and Tx (64-66 GHz) channels, separately. Meanwhile, this dual-mode cavity becomes the interconnect between Rx/Tx channels and help to realize good isolation without using conventional T-junction. In the measurement, each filter designed for Rx and Tx channels exhibits excellent performance. Channel-to-channel isolations better than 35 dB across the Rx band and better than 32.5 dB across the Tx band are also obtained.