In the present study, the soot particles produced from diffusion flames burning biodiesel fuel were thermophoretically sampled and the carbon nanostructure of soot particles were imaged using a high resolution transmission electron microscopy (HRTEM). The HRTEM images of soot particles were then quantitatively analyzed using a digital image processing algorithm developed and implemented as part of this work. The HRTEM interpretations with an aid of image processing support feasibility of use of the developed image processing technique for carbon nanostructure quantification.