A new design technique for flip-chip structures with an optimized coplanar waveguide (CPW) is proposed. One of the features of this technique is that wide-gap CPW patterns are formed to reduce the excessive capacitance between the two signal lines in the flip-chip structure. Thus, it is possible to achieve a return loss in excess of 28 dB up to 58 GHz. Another feature is that the design method is improved by using two-dimensional (2D) cross-sectional analysis to optimize the parameters of the flip-chip structure. The design turn-around time is at least one-third that of the method using only three-dimensional (3D) electromagnetic field analysis. By applying our method to the design of a wide-band photoreceiver, we estimated a 3-dB down bandwidth up to 60 GHz. These results show that our proposed techniques are useful for millimeter-wave or wide-band communication systems. © 2000 John Wiley & Sons, Inc. Int J RF and Microwave CAE 10: 289–295, 2000.