This study investigated the use of a blend of waste vegetable oil and diesel fuel in a diesel tractor engine by comparing the resulting particulate matter (PM) collected on a filter with the PM of diesel fuel. To this purpose, Fourier transform infrared (FTIR) and elemental analyses were carried out and the PM collected on the filters was examined via scanning electron microscopy (SEM). The study also investigated the overall morphology of soot particles from the blend of 75% waste cooking oil (WCO) + 25% diesel (B75) in a diesel engine at three different loads (75%, 50% and 10%) compared with the morphology of the soot particles from diesel fuel (B0). The FTIR spectra of the B75 fuel exhibited the characteristic regions of oxygen bonds. Compared to the B0 fuel, the increased oxidation from the excess oxygen in the B75 fuel reduced the size and number of soot particles. It was also verified that the degree of unsaturation was related to the oxygen content of the B75 fuel. The FTIR spectra of the B75 fuel showed a C = O tensile band methyl ester in 1734 cm(-1) and C-O bands at 1214-1362 cm(-1). Furthermore, similar FTIR spectra were seen for the diesel fuel (B0) and the B75 fuel. The results showed a significant reduction in soot particle emissions with the WCO-diesel blend. The results of this research have enabled a better evaluation of the effects of different fuel usage on diesel engine combustion performance and emission characteristics.