In this paper, an optimized circular four rings 16-level quadrature amplitude modulation (C-16QAM) is designed to mitigate the I/Q imabalance and phase noise in 60 GHz millimeter-wave six-port direct down-conversion receiver. This modulation technique is performed through the optimization of both, Euclidean distance between symbols and the gap between the rings. The experimental results show that the proposed constellation can achieve lower bit error rate (BER) compare to the typical square 16-QAM and other C-16QAM modulations reported in the literature.