Abstract: This paper investigates the possibility of the cottonseed-diesel oil, which was mixing cottonseed oil with diesel oil,used in diesel engine. A mathematical model on the main performance of different proportions of cottonseed-diesel oil was establised. The test-bed test and production test have been carried out to study the power characteristics,economical characteristics, effective efficiency, and the wear and tear of the main parts of the engine. Synthetically analysing the test results test, it comes to the conclusion that the cottonseed-diesel oil can be used as substitute fuel of the diesel engine.