Abstract: Some product oil such as diesel will be clotted if it is stored in winter tank, so that the final temperature of the oil at natural cooling has to be calculated to determine whether the tank is insulated/heated or not.Therefore, the authors derivate the functional relations of the final temperature of diesel by mathemat-ic model and calculate the actual value with trial method.It is considered that the functional relations can offset against the disability of other algorithmic methods in this subject, because each result obtained is ratiocination-based value in accordance with all known parameters, which is less discrepancy and suitable for actual application of engineering.