A method for determining major characteristic parameters of naphthas by capillary gas chromatography was developed to identify the individual hydrocarbons in naphtha and determine the hydrocarbon-type PNA value, the average molecular weight, the carbon-to-hydrogen ratio, the density and boiling point range of naphtha. The method can also be used to analyze other light fractions of petroleum such as natural gas liquid and by-products from petrochemical processing such as raffinate oil. The data obtained by the method had a good agreement with those by other methods. The method was not affected by surroundings conditions when determining boiling point range of light petroleum fractions and can provide with more detailed information. The recovery percentage was 100% when the method used to measure boiling distribution of light fraction. The method was utilized to predict the pyrolytic effect of different naphthas for ethylene production. The prediction had an agreement with the results that were obtained by pyrolysis of naphthas on the Bench Scale Pyrolysis Apparatus (BSPA) made by M.W. Kellogg Company. (c) 2005 Published by Elsevier B.V.