Ultrafine (<100 nm) ash particles in three coal fly ashes (CFA) produced by the combustion of three U.S. coals have been examined by high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), and electron diffraction. These ultrafine particles, either as primary ash particles derived directly from coal minerals or as secondary products of decomposition and vaporization-condensation processes, show quite different morphologies, compositions, and microstructures as compared to particles in coarser, micrometer-size fractions previously examined by CCSEM. An eastern U.S. bituminous CFA sample shows abundant discrete crystalline particles rich in Fe, Ti, and Al in its ultrafine ash fraction, and crystalline phases down to 10 nm size have been identified. Western U.S. low-rank CFA samples contain considerable amounts of alkaline-earth element aggregates in the form of phosphates, silicates, and sulfates and mixed species. Most of them show crystalline or crystalline plus amorphous characteristics. All three ultrafine samples also exhibit carbonaceous particles in the form of soot aggregates with primary particle size typically between 20 and 50 nm. In the western low-rank ultrafine CFAs, these carbonaceous soot particles were typically mixed or coated with multi-element inorganic species.