Ca3Co4-xCuxO9+delta (X = 0.00, 0.05, 0.07, 0.10 and 0.15) samples were prepared by conventional solid-state synthesis and their thermoelectric properties were systematically investigated. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. Ca3Co3.85Cu0.15O9+delta had the highest power factor of 2.17 mu W cm(-1) K-2 at 141 K, representing an improvement of about 64.4% compared to undoped Ca3Co4O9+delta. Magnetization measurements indicated that all the samples exhibit a low-spin state of cobalt ions. The observed effective magnetic moments decreased with increasing copper content. (C) 2013 Elsevier Ltd. All rights reserved.