This paper is the first part of general study started at Assiut Universitydealing with effects of mineral oil on reinforced concrete structures. Thisstudy focused on the impact of used engine oil on the compressive strengthof concrete. Compressive strength of concrete cubes soaked in usedmineral oil for six months were compared with that of oil-free concretecubes. Five different concrete mixes were adopted to investigate theaggressiveness of mineral oil on low and high strength concrete.Furthermore, this paper describes an investigation into the effect of oilsaturation on concrete frame foundations used for large machines ofCement Assiut factory-CEMEX. It is concluded that evaluation of theaggressive effect of mineral oil on concrete by comparing the strength onoil-free concrete and oiled concrete at the same age experimentally is theoptimal and most reliable of the existing method. The aggressiveness of oilis very clear for low and high strength concrete and the reduction incompressive strength is 17, and 11.8 percent respectively. Additionally,based on the field study, degree of oil saturation in concrete plays animportant role in compressive strength.