The electrical and magnetic properties of one-dimensional calcium iridium oxide Ca5Ir3O12 are investigated. A weak ferromagnetic transition has been found at 7.5 K through magnetic susceptibility measurements. At the same temperature, a lambda-type specific heat anomaly has been observed. The effective magnetic moments in the paramagnetic temperature range and the magnetic entropy due to the magnetic transition indicates that the tetravalent and pentavalent Ir ions exist in the ratio of 1:2. Another lambda-type anomaly has been observed at 105 K in the temperature dependence of the specific heat. The electrical conductivity shows one-dimensional Mott variable-range hopping conduction behavior. (C) 2003 Elsevier Science Ltd. All rights reserved.