We studied engine-out soot samples collected from a heavy-duty direct-injection diesel engine and port-fuel injection gasoline spark-ignition engine.The two types of soot samples were characterized using Raman spectroscopy with different laser powers.A Matlab program using least-square-method with trust-region-reflective algorithm was developed for curve fitting.A DOE (design of experiments) method was used to avoid local convergence.The method was used for two-band fitting and three-band fitting.The fitting results were used to determine the intensity ratio of D (for Defect or Disorder) and G (for Graphite) Raman bands.It is found that high laser power may cause oxidation of soot sample,which gives higher D/G intensity ratio.Diesel soot has consistently higher amorphous/graphitic carbon ratio,and thus higher oxidation reactivity,compared to gasoline soot,which is reflected by the higher D/G intensity ratio in Raman spectra measured under the same laser power.