Abstract: A simple and efficient straight in-line fuel pretreatment device is described, utilizing dwell time, magnet orientation and magnetic field strength to enhance the combustion properties of all types of hydrocarbon fuels, both liquid and gaseous fuels. A plurality of pairs of magnets, more with facing opposite poles and fewer with facing like poles, are aligned within the fuel conduit to a burner. The resulting magnetic field strength must be at least 500 gauss, preferably 500-1000 gauss, with dwell times in the range of about 0.1-1.0 second. Also disclosed is a method for pretreatment of combustion fuel which comprises passing a quantity of the combustion fuel through such magnetic field for a dwell time sufficient to enhance the combustion properties of the fuel. The pretreatment device can be made in standardized sizes for various types of commercial, light industrial and residential burners and combustion units.