This paper presents a 36-element low-profile half-duplex K-band T/R phased array as a solution for the platform with limited aperture and space for the phased array. With the solution of micro-assembly presented in this paper, the height of the array is only 12.3 mm. To improve the phased array's performance in ultra-small aperture, multi-channel GaAs MMICs have been applied in this work. The array's scan angle could achieve ±50 • in both azimuth (H-plane) and elevation (E-plane). The EIRP and G/T of the array are 51dBm and-12.5dB/K respectively at the bandwidth from 21.5 GHz to 22 GHz. INDEX TERMS GaAs, half-duplex, K-band, low-profile, micro-assembly, multi-channel MMIC, phased array, ultra-small aperture.