AbstractAt present, flat cables for the under‐carpet use have a shielded structure to improve crosstalk characteristics. However, unshielded cables are advantageous when a very thin structure is desired. This paper is concerned with crosstalk attenuation characteristics of unshielded flat cables laid in close proximity to the ground. In this case not only the direct crosstalk due to electrostatic and magnetic couplings between conductors but also the indirect crosstalk via a path along the line to the ground must be considered. By defining the unbalance angle θ to express the degree of impedance unbalance to the ground, the coupling impedance between the longitudinal line and the real line was calculated and formulas to calculate the impedance unbalance to ground and the indirect crosstalk attenuation were derived under the assumption of a perfect conductor for the ground. Next, using these formulas the indirect crosstalk attenuation and the impedance unbalance to ground were studied as functions of the cable structure and actual cable layout such as the pair distance and the ground separation. As the result, the indirect crosstalk can not be improved by increasing the pair distance, and the impedance unbalance to ground is approximately proportional to the crosstalk attenuation. The results obtained are useful in the crosstalk design of unshielded flat cables.