The increasing consumption of fossil fuels and the high emission of exhaust gases have encouraged researchers to explore different approaches to reduce its consequences. Apart from focussing on the engine components, modifications on the fuel molecules can also deliver huge improvement. Using magnetic fields on the fuel line becomes one of the promising alternatives to give a better performance of the engine and produce less hazardous emissions. Previous researchers have proven that the magnetic field could enhance combustion rate by influencing the fuel molecules. After the magnetic treatment, hydrogen atoms in the hydrocarbon fuel tend to react better with oxygen molecules, thus creating more improved combustion. However, the results were varied among the researchers with huge ranges, especially in the rate of fuel-saving and emissions depending on the experiment’s setup. While many previous researchers have reported significant improvement in engine performance with the use of the magnetic fields on the fuel line, some reported insignificant effects. Despite the promising potential, this method has not received much attention from both automotive manufacturers and users. The objective of this paper is to discuss the previous studies regarding influences of the magnetic field to the hydrocarbon fuel and engine’s output. Based on the up-to-date research findings, discussion and explanations consisting of molecular reactions, important factors in influencing the changes in the engine performance and exhaust emissions are to be discussed.