Precise analytical methods for characterizing diesel fuel yielding the lowest particulate emissions were developed. The methods consist of preparative-scale high pressure liquid chromatography (HPLC), field ionization mass spectrometry (FIMS), analytical-scale HPLC, and carbon-13 nuclear magnetic resonance spectrometry (13C-NMR).A diesel fuel was first separated into an aliphatic fraction and an aromatic fraction by semipreparative-scale HPLC. Then, the aliphatic fraction was analyzed by FIMS and the spectrum was compared with that of the whole fuel. The aromatic fraction was analyzed by analytical-scale HPLC to obtain the chromatogram of the aromatic hydrocarbons with a high S/N. In addition to these analyses, the fuel was analyzed by 13C-NMR to obtain the concentration of the carbon atoms of the straight chain, branched chain and aromatic-ring in hydrocarbons.These results were found to offer the composition of the diesel fuel according to the molecular formula and partial structure of hydrocarbon. That is, these results explained the cause for the difference in particulate emissions from the fuels with the same H/C and back end fraction. The two parameters had already been found as the parameter showing the tendency of the fuel to form soot and the parameter closely related to soluble organic fraction (SOF), respectively, in the paper of SAE 971605.