Viscosity and thermal conductivity are two temperature dependent properties of matter. In most of the problems of fluid mechanics, the viscosity and thermal conductivity of the fluid were assumed to constant. However it is known that these physical properties can change significantly with temperature.When the effect of variable viscosity and thermal conductivity are taken in to account, the flow characteristics are substantially changed from that of constant cases. Most fluids however have temperature dependent properties, and under circumstances where large temperature gradients exist across the fluid medium, fluid properties often vary significantly. Under many conditions, ignoring such variations may cause serious in-accuracy in estimating heat transfer rates. So, here in this book we are considering some heat and mass transfer problems of fluid mechanics where the effects of variable viscosity and thermal conductivity are observed under different flow conditions. The book will be helpful for researchers working in the field of heat and mass transfer flow.