Magnetic field applied to fuel can alter its characteristics in terms of forces that hold the hydrocarbons together. This principle was used to investigate the impact of incorporating a magnetic tube in the fuel intake in a diesel generator on specifically the energy performance and pollutant emissions. A diesel generator was fitted with a magnetic tube in the fuel intake, which had valves to switch from without magnetic tube case to with magnetic tube case. The diesel generator was operated at constant speed of 1800 rpm at idle condition, 25% and 50% loads, respectively. Additionally, two real diesel cars were deployed with magnetic tube and their fuel consumption compared with that of a car without magnetic tube. With application of magnetic tube, the brake specific fuel consumption and fuel consumption were decreased by an average of 3.5% and 15%, respectively, while the brake thermal efficiency was improved by approximately 3.5%. The particulate matter, carbon monoxide, hydrocarbons and carbon dioxide emissions reduced in the range of 21.9–33.3%, 5.4–11.3%, 29.4–64.7% and 2.68–4.18%, respectively. Both the total PAH concentrations and total BaPeq concentrations can be reduced by about 63%, 45% and 51%, respectively for idle condition, 25% and 50% loads, respectively. These results show that application of magnetic tubes in the diesel engine is a promising technology in pollutant reduction and energy saving.