An investigation on the morphology, internal microstructure, and fractal geometry of particulate matter (PM) was conducted for a 1.7-L light-duty diesel engine. The experimental and analytical instruments used in this study include a unique thermophoretic sampling system, a high-resolution transmission electron microscope (TEM), customized digital image processing/data acquisition system, and a Raman scattering spectrometer that have been extensively used at Argonne National Laboratory. During experiments, diesel particulates were directly collected from the exhaust stream. In the morphological observations using TEM, graphitic crystallite structures were found from these light-duty diesel particulates, particularly at high engine loads. In subsequent analysis, Raman spectra revealed details of these graphitic structures quantitatively: the collected diesel PM was quite similar in atomic structures to typical graphite, and the degree of its crystalline structures increased as the engine load increased. in PM size measurements, mean primary particle diameters (d(p)) are in a range of 19.4-32.5 nm and radii of gyration (R,) in a range of 77.4-134.1 urn, respectively, across the engine operating conditions of 780-4000 rpm and 0-100%, loads. From the fractal analysis, smaller fractal dimensions (D-f) (1.46-1.70) were measured for the light-duty diesel engine used in this study, compared to those of heavy-duty diesel particulates that ranged in 1.80-1.88. This result implies that more stretched chain-like particles were produced from the light-duty engine under the diffusion-limit formation mechanism. The database on the physical dimension and fractal geometry of light-duty diesel PM obtained from this study offers data for the future standards of diesel PM sizes and development of advanced diesel PM emission control technologies. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.