Diesel particulate with different aerodynamic sizes was collected from a medium-duty diesel engine using a micro-orifice uniform deposition impactor (MOUDI). High-resolution transmission electron microscopy (HRTEM) and thermogravimetric analysis (TGA) were used to investigate the volatility, morphology, inner nanostructure, and oxidation behavior of the particulate in four aerodynamic size bins of <56, 100-180, 320-560, and 1000-1800 nm. The results indicate that the particle volatile fraction and fractal dimension first decrease and then increase with the increase of the aerodynamic size, while the average diameter of primary particles increases with the aerodynamic size. Quantitative analysis of the HRTEM images indicates that the primary particles in size bins of 100-180 and 320-560 nm have a more orderly nanostructure compared to those in the the other two size bins. The results of the oxidation experiment indicate that in the size bins of <56 and 1000-1800 nm, the particulate samples exhibit stronger oxidative reactivity than those in the other two size bins., which could be related to the different particulate volatility, morphology, and nanostructure in the different size bins.