In many engineering projects which are carried out in rock should deal with weathering mechanisms that weaken the rock mass as a consequence of the changes in rock material. Mafic and ultramafic rocks are known to have sound and stiff structure. This situation is valid in unweathered and slightly weathered stages. By the increase of weathering these rocks show sharp decrease in physico-mechanical properties. This situation is observed in Bursa (Turkey) on diorites as a mafic rock and on dunites and pyroxenites which are the two major ultramafic rocks. Dunites and pyroxenite also tend to serpentinize in every stage of weathering. The change of the engineering properties of these rocks in varying weathering grades were determined by field and laboratory studies. Scan-line studies as proposed by ISRM (2007) and Schmidt hammer tests are carried out during field surveys. Mineralogical evaluations, chemical indices, physical properties, uniaxial compressive strengths and slake durability tests were performed during the laboratory studies. According to the field and laboratory studies the weathering profiles of these rocks were determined. It was concluded that serpentinization controlled the engineering properties of dunites and pyroxenites by increasing weathering grades. On the other hand the increase of fractures and reduction of grain sizes are the main factors influencing the engineering properties of diorites due to weathering.