Abstract: Field desorption mass spectrometry has been used to determine the molecular weight of complex triglycerides in natural oils and fats. This method provides the basic information about components of the oils and fats.The distribution and content of fat acids which are made of different number on carbon atoms and the number of average double bonds in the triglycerides can be calculated by means of data processing. But this method can not provide the position of these fat acids in triglycerides,