The different kinds of sliding surfaces like clutches, mechanical seals and breaks are susceptible to a surface damage called thermoelastic instability. In this research, the effect of surface texture on the thermoelastic instability is investigated under mixed lubrication regime. A computer program is employed to numerically generate three different surface roughness patterns i.e. transverse, longitudinal, and isotropic. A model consisting of a surface with high thermal conductivity and a rough surface with low thermal conductivity is considered. The flow factors which are correction factors to the Reynolds equation for considering the surface roughness pattern are employed to study the effect of surface pattern on the thermoelastic instability. An algorithm is developed to regenerate the thermoelastic instability results in the published literature and then is used to find the critical speed for three types of surface textures (longitudinal, transverse and isotropic) beyond which thermoelastic instability leading to the formation of hot spots is likely to occur. This algorithm is then developed to consider surface texture for bearings with different thickness and for lubricants with different viscosity. Finally it is shown that the critical speed for longitudinal surface pattern is higher than two other surface patterns for bearings with different thickness and for lubricants with different viscosity. Another research that was done accompanied by this thesis is to study the effects of different methods of surface machining on helical gears operation under mixed lubrication regime. The surface pattern parameter, which is the indicator of roughness direction, and the rms of the roughness profile are the inputs to this program and the roughness height matrices for three types of surface textures are the outputs. The produced surface is then used in an analytical model to predict the performance of helical gears. In this model which is called the load-sharing model, the pressure is distributed along the contacting surfaces of helical gear teeth. It is shown that in the transverse surface pattern a larger portion of the load is supported by roughness comparing to two other surface patterns and the friction coefficient is more than two other surface patterns too. Also experimental tests for comparing the wear volume and the friction coefficient in longitudinal and transverse surface patterns are conducted using the pin on disk wear test. These tests were done for different forces, different speeds and different distances using ST37 steel as disks material and SAE 10W-40 as lubricant. It is shown that the wear volume in transverse surface pattern is much higher than the wear volume for a surface with longitudinal roughness pattern. This result is in complete agreement with the findings of the last two sections. Keywords surface pattern, surface roughness, thermoelastic instability, helical gear, tribology