In this present paper, we studied about thermal stability in a Newtonian fluid. A detailed account of thermal convection in a Newtonian fluid layer in the presence of magnetic field has been given by Chandrasekhar [11]. Stokes [79] has formulated the theory of couple-stresses in fluids. The theory due to Stokes allows for polar effects such as the presence of couple stresses and body couples. The theory has been applied to the study of some simple lubrication problems. According to the theory, couple-stresses are found to appear in noticeable magnitudes in fluids with very large molecules. Since the long chain hyaluronic acid molecules are found as additives in synovial fluid, Walicki and Walicka [80] modeled synovial fluid as a couple-stress fluid in human joints. A human joint is a dynamically loaded bearing which has particular cartilage as the bearing and synovial fluid as the lubricant. A synovial fluid is the natural lubricant of joints of the vertebrates. The synovial is obtained from the hyaluronic acid, a fluid of high viscosity, near a gel. Normal synovial fluid is clear or yellowish and is a non- Newtonian, viscous fluid. The shoulder, hip, knee and ankle joints are loaded with the synovial fluids and joints have a low friction coefficient and negligible wear.