Stick-slip has been considered to be a mechanism of shallow focus earthquakes along pre-existing faults, and thus laboratory studies relevant to stick-slip may lead to a better understanding of the mechanisms of the earthquakes and eventually to an effective prediction and/or control of them. This paper deals with the effects of various simulated gouges on the sliding behavior of rocks with special emphasis on how fault gouge alters potentially unstable behavior of a brittle rock. About 150 specimens of Tennessee sandstone with various simulated gouges along a 35 ¿ percut are deformed dry in a triaxial apparatus at room temperature, with shortening rate of about 2¿10-4cm/s and at confining pressures at 300 MPa. Tennessee sandstone is a brittle and strong rock (quartz-cemented quartzose sandstone) which consistently shows stick-slip without simulated gouge at confining pressures about 30--40 MPa. Sliding behavior with a gouge layer depends markedly on the material used to simulate the gouge. Experimental results from monomineralic gouges show that Mohs' hardness of a mineral is a useful parameter to predict the general sliding behavior. The most unstable sliding is associated with gouge composed of minerals with intermediate hardness (dolomite, anhydrite, and calcite); these gouges undergo the transition from stable sliding to slick-slip at pressures of 50--80 MPa. Gouge composed of hard mineral such as quartz or feldspar behave more stably, but stick-slip occurs at pressures about 250 MPa. Only stable sliding is observed for gouge composed of soft minerals like halite. Although anhydrite gouge shows violent stick-slip at 100 MPa pressure, whereas only stable sliding occurs for quartz at the same pressure, when they are mixed, the behavior changes systematically with composition. The inclusion of only 10% anhydrite permits stick-slip, and hence even a minor constituent mineral of fault gouge may alter the sliding mode. For gouge composed of two distinct layers, the more stabilizing member of the two dominates the behavior, and the behavior is considerably different from that of homogeneously mixed gouge with the same bulk composition. This suggests the significance of internal structures of natural fault zone in under standing the effects of fault-zone materials on the fault motion. The gouge composed of calcite or a mineral harder than calcite all show high and approximately the same ultimate frictional strength, with an exception of fluorite. The frictional strength is intermediate for fluorite, galena and barite gouges, and it is low for halite gouge.