The imported oil has had an enormous impact on so many people, so rapidly in so many ways and in so many places around the world. At the same time we must not forget the need for clean air, clean water, clean fuel and biodegradable, renewable materials. Hazardous pollutants that lead to atmospheric pollution have many sources and automobile’s exhaust emission is one of these. Therefore, it needs to be taken into consideration seriously the replacement of fossil fuels with renewable fuels. More affordable and longer lasting, renewable fuels not only resurrect the economy but also save the environment before it’s too late. This paper will examine the use of diesel-soy oil mixtures in diesel four-stroke engine. The mixtures that have been used are the following: diesel-5% soy oil, diesel-10% soy oil, diesel-20% soy oil, diesel-30% soy oil,  diesel-40% soy oil, diesel-50% soy oil. The fuels were submitted in different temperatures 10οC, 20οC, 30οC, 40οC, 50οC, 60οC. For those mixtures and for every fuel temperature the gas emissions of carbon monoxide (CO), hydrocarbons (HC), nitrogen monoxide (NO) and smoke have been measured. The fuel consumption and the gas emission temperature have also been measured.