Performance of a single-cylinder, water-cooled, direct-injection diesel engine on dual-fuel operation with Jatropha oil (JO) as pilot fuel and orange oil as primary fuel was evaluated. Constant load test at different power outputs was conducted at the rated speed of 1500 r/min with varying orange oil quantities. The loads were fixed as 20 per cent, 40 per cent, 60 per cent, 80 per cent, and 100 per cent. In dual-fuel operation with orange oil induction, the thermal efficiency of JO was increased mainly at high power outputs. Maximum thermal efficiency with JO was found as 29 per cent at 31 per cent of orange oil induction at 100 per cent load. Smoke was reduced significantly with all orange oil induction rates at all power outputs in dual-fuel operation with JO. It was reduced from 4.4 to 3.3 BSU (Bosch Smoke Units) with JO at the maximum efficiency point at 100 per cent load. HC emissions were increased further at all power outputs in the dual-fuel mode with all rates of orange oil induction. Dual-fuel operation increased the ignition delay of JO. However, peak pressure and energy release rates were improved in the dual-fuel operation with orange oil induction. In general, dual-fuel operation with orange oil as inducted fuel with JO as pilot fuel showed inferior performance and emissions at part loads. It is concluded that the JO as pilots fuel and orange oil as the inducted fuel could be used in diesel engines with reduced smoke levels and improved thermal efficiencies with no major detoriation in performance. © 2011 Authors.