Abstract: With the rapid developement of petroleum industry in our country a great amount of storage tanks has been constructed in the oil field, refineries, storage bases. The volume of oil tank is increasing oweing to technique progress, and investment per volume of tank decreasing. In the paper are presented the variation rules between the tank investment and volume, corresponding mathmatic model. Through the analysis and discussion of the math model application the author indicated that, (1). it is for one type of oil to discuss the correlation between the investment and the volume, (2). the investment is different for the same volume of oil tank if tanks are located in various regions, and the establishment of math model must be according to the practical conditions.