In this article, the particulates formed by the combustion of diesel, 20% and 50% biodiesel blended in diesel (denoted as B20 and B50, respectively), and biodiesel were collected. The micromorphologies of the particulates were observed by scanning electron microscopy. Furthermore, the small‐angle X‐ray scattering technique was used to analyze the gyration radius, size distribution, interfacial characteristics, and fractal characteristics of the particulates. The results show that the size of the particulates formed by combustion of the fuels was concentrated between 10 and 35 nm and obeyed a Gaussian distribution. Moreover, the addition of biodiesel to diesel led to smaller particulate sizes for the biodiesel/diesel blend: the mean radii of the particulates for diesel, B20, B50, and biodiesel were 11.5 nm, 10.8 nm, 10.2 nm, and 9.7 nm, respectively. In addition, an interfacial layer was observed between the particulates, and the average interfacial thickness of the particulates was between 1.54 and 1.89 nm. The mass fractal dimension of the particulates was between 0.2 and 2.5, and the surface fractal dimension of the particulates was between 2.1 and 2.9. The addition of biodiesel to diesel resulted in a more compact particulate structure and the rougher surface. © 2019 American Institute of Chemical Engineers Environ Prog, 38:e13146, 2019