The effect of biodiesel produced from waste cooking oil on the particulate emissions of a direct injection (DI) diesel engine was investigated experimentally and the results were compared with two diesel fuels, namely, an ultra low sulfur diesel fuel with less than 10-ppm-wt of sulfur (ULSD) and a low sulfur diesel fuel with 400-ppm-wt of sulfur (LSD). For each fuel, the number and mass based particle emissions, as well as the particle volatility, were evaluated and compared. The particulate mass emissions were measured with a tapered element oscillating balance (TEOM) and further divided into different size bins using a micro-orifice uniform deposition impactor (MOUDI). The particle number concentration and size distribution were measured with a scanning mobility particle sizer (SMPS). The size-segregated samples collected with the MOUDI were further analyzed with a thermogravimetric analyzer (TGA) to obtain the mass of volatile substances in each size bin. The SMPS was further connected in series with a thermodenuder (TD) to obtain the number concentration and size distribution of non-volatile particles, and hence the number concentration and size distribution of the volatile particles. The results indicate that the biodiesel could effectively reduce the particle mass and number concentrations, including the volatile substances, in all the measured size range, compared with LSD. Compared with ULSD, there is also a reduction in the particle mass and number concentrations, however, a higher concentration of volatile substances was found with the use of biodiesel, which should be a concern in the application of this fuel.