Bunker C fuel oil may be taken as a conc. solution of asphalt as a solute. It may be assumpt that there will be unalogical relationship between cone. solution and solute in regological behavior. Investigation was carried out to fiud out the -opitimum preheating temperature. The following results were obtained: the colloidal structure bunker C fuel oil undergoes a transition at around the softening point of the solute asphalt: and the flow charactor changes from non-Newtonian flow to Newtonian as well as its activation energy is memarkably reduced at around softening point of the solute asphalt for the purpose of the improvement of flow charater of Bunker C fuel oil, the preheating must be done above the softening point of a solute asphalt.