Experiments were conducted to evaluate the combustion, performance and emission parameters of a single cylinder, four stroke, air cooled, direct injection diesel engine, fueled with used transformer oil (UTO) and six of its diesel blends on varying the UTO concentration from 10% to 60%, at a regular interval of 10% by volume basis. The results were analyzed and compared with diesel operation. Increase in thermal efficiency with significant improvement in reduction of smoke was observed for UTO and its diesel blends compared to diesel. NO emissions were higher for UTO and its diesel blends than that of diesel. Ignition delay was marginally shorter for UTO and its diesel blends than diesel. (C) 2012 Elsevier Ltd. All rights reserved.