Combustion-generated fine particulate matter is linked to a variety of adverse effects on the environment and human health. Characterizations of the size, shape, structure and chemical composition of such harmful particulates are essential to monitor, regulate and model their transport, toxic, optical, chemical and deposition properties. In this study, particulates emitted from three combustion sources are investigated in order to identify key similarities and differences in their physical characteristics and compositions. Specifically, a 6-cylinder medium duty compression-ignition engine burning number 2 diesel fuel, a single-cylinder spark-ignition engine fueled by isooctane and a local utility power plant burning coal/woodchip mixtures are considered. Direct sampling methods are employed to extract particulates from the exhaust plumes onto microscope grids without any sample preparations or manipulations. The samples are then analyzed with high-resolution transmission electron microscopy, energy dispersive x-ray spectroscopy, and electron diffraction method. Physical sizes and morphologies, chemical compositions, crystalline properties and fine internal structures are determined and compared. The present experimental findings provide scientific guidance for the identification of particulate source in heavily polluted areas and for the improvement of emission reduction strategies.