In this paper, we have presented a design of onchip millimeter-wave 3-dB quadrature coupler that utilizes the coupled slow-wave coplanar waveguide (CS-CPW). The designed CMOS coupler covers the whole E-and W-band and occupies a silicon area of only 0.0115 mm 2 which is significantly smaller compared to the conventional microstrip-line-based Lange couplers. Measurement of the quadrature coupler shows a -3.5 dB through and a -4.4 dB coupling at 90 GHz. A less than ±1-dB amplitude and a ±4 • phase errors from 55 to 110 GHz are recorded.