Abstract-A 400 GHz monolithic leaky wave antenna (LWA) is presented in this paper. The proposed LWA, constructed by the unit cell with multiple structural parameters, is regarded as the on-chip microstrip with perforation on the signal trace and the ground plane. A hybrid full-wave eigenvalue method theoretically extracts the complex propagation constants of first higher-order mode (EH 1 ) of the perforated microstrip to improve the unit cell design. The extracted results also assist in realizing the differential feeding network to excite the leaky mode of the proposed antenna in high efficiency. A 400 GHz LWA prototype is designed and fabricated in CMOS 0.13 µm 1P8M process. The on-chip experiments show the measured input return loss including the effects of the contact pad lower than 10 dB from 380 GHz to 420 GHz. The measured antenna gain is higher than 0.8 dBi and has a maximum value of 1.3 dBi at 400 GHz. From 390 GHz to 405 GHz, the measured main beam is at 33 • to 43 • from broadside, indicating good agreement with the calculated results.