To utilize bio-butanol as an alternative diesel fuel, the effect of butanol isomer, where 1-butanol, 2-butanol and isobutanol were studied except for tert-butanol, on the combustion characteristics and exhaust emissions of butanol/gas oil blend was investigated using a DI diesel engine without modification of engine parameters. First, to understand the effect of butanol content on the diesel combustion, engine test was carried out using blends of 1-butanol which contents were 10 to 50 mass%. With increasing 1-butanol content, the Smoke emission reduces although the ignition delay gets longer and the HC and CO emissions increase especially at low load. The engine operation is stable except for full load with 1-butanol 50 mass% blend. From the above experimental results, butanol isomer blending ratio is set to 40 mass%. In order of 1- butanol/gas oil, 2-butanol/gas oil, isobutanol/gas oil, the ignition delay gets longer, that is depended on the molecule chain structure of butanol isomer, and the HC and CO emissions increase especially at low load. However, there is few effect of butanol isomer on BTE and the NOx and Smoke emissions at middle and high load conditions. It is concluded that 1-butanol is best for gas oil blending use among three isomers.