This paper presents a four-channel bi-directional core chip in 65 nm CMOS for Ka-band phased array Transmit/Receive (T/R) module. Each channel consists of a bidirectional gain block (BDGB), a 5-bit step attenuator, a 5-bit phase shifter. The chip size is 4.1 × 1.2 mm2 including pads. The phase and attenuation coverage is 360° with the LSB of 11.25°, and 31 dB with the LSB of 1 dB, respectively. The Tx mode reference-state gain in each channel is 13.3 dB and the Rx mode gain is 16.5 dB including the 4-way power combiner. The output P1 dB in Tx mode is > 1 dBm and the noise figure in Rx mode is 7.5 dB. To authors' knowledge, this is the smallest size per channel Ka-band core chip in CMOS technology with bidirectional operation and enable dual polarization mode.