The greenhouse effect, thought to be responsible for global warming, is caused by gases accumulating in the earth＇s atmosphere. Carbon dioxide, which makes up half of the gas accumulation problem, is produced during respiration and combustion processes. This paper provides an outline of the carbon cycle for rapeseed oil-derived fuels. Plant processes, fuel chemistry and combustion are examined with respect to carbon. A diagram is presented to interpret the information presented graphically. A comparison of carbon dioxide emissions from the combustion of rapeseed oil biodiesel and petroleum diesel is made. Complete combustion converts hydrocarbon fuels so carbon dioxide and water. The carbon cycle consists of the fixation of carbon and the release of oxygen by plants through the process of photosynthesis, then the recombining of oxygen and carbon to form CO2 through the processes of combustion and respiration. The carbon dioxide released by petroleum diesel was fixed from the atmosphere during the formative years of the earth. Carbon dioxide released by biodiesel is fixed by the plant in a recent year and is recycled. Many scientists believe that global warming is occurring because of the rapid release of CO2 in processes such as the combustion of petroleum diesel. Using biodiesel could reduce the accumulation of CO2 in the atmosphere.