Straight run diesel and motor oil are hydrocarbons of same group, the difference is their molecular weight, so through catalyzing cracking treatment, motor oil can be converted in to diesel. With rational design and proper treatment, this diesel is comparable to common diesel, the cetane number is high and this oil has stronger power especially suitable for heavy trucks fuel. But to ensure oil quality, the design of this kind of unit is very important, cracking shall be equably to avoid burnt of oil and change molecular chains and reduce the density of the oil. For example the distillation column size, height, design of catalyzing unit, treatment procedure etc. are very important which need experienced engineers make a rational design through good knowledge and practical operation experience. During processing, gasoline and diesel could be obtained through different distillation temperature ranges. 40-190c is gasoline fraction range, and 250-330c is diesel fraction range. After pyrolysis and cooling, Approx 85% liquid is similar to diesel and able to meet the specification of -15# diesel. They’re able to be used as industrial or civil-use fuel, even auto driven fuel. Motor oil is from a fraction after diesel in oil refining industry, they’re belong to the same petroleum family having different molecular weight, it is very feasible to produce diesel.