The main objective of this research work is to study, experimentally, the effects of alcohol blending of diesel and biodiesel fuels on the mechanical and thermochemical characteristics of a diesel engine. This experimental study was carried out on a single cylinder four-stroke and air-cooled diesel engine As a first step, we have investigated the effect of adding small quantities of different types of alcohol on the diesel engine performances. In this paper, we present only the results obtained for 2.5% of ethanol added to pure diesel fuel. The comparison was made between pure diesel D100 and a mixture between 97.5% of pure disesl and 2.5% of ethanol (D97.5E2.5). The tests were carried out under four different loads 1/4, 1/2, 3/4 and full load. Experimental results showed a decrease in the specific fuel consumption (SFC) of the D97.5E2.5 compared to the D100. The decay margin increases as the load increases and starts from 4.9% at 1/2 load up to 27.6% at full load for an engine speed at the vinicity of 2100 rpm . The effect of the ehanol addition on engine power (BTP) was remarkable. An increase in the BTP was observed under full load and for all engine speed range. İn the case of other loads, the increase on BTP was observed only at speeds above 2500 rpm.