Abstract Channel diameter is one of the most important parameters of a heat exchanger especially for a highly viscous fluid-flow. Narrow channel heat exchangers are believed to have better energy efficiency due to elevated heat transfer characteristics. Heat transfer and Fluid-flow behaviors of Automatic Transmission Fluid ( ATF) have been experimentally investigated in a closed loop integrated thermal wind tunnel test facility using wavy finned Minichannel Heat Exchanger (MICHX). The experiment was conducted by varying the ATF Reynolds number from 3 to 30. The flow friction factors in minichannel were evaluated. For a fully developed laminar flow the friction factors were evaluated considering fluid viscosity effects due to temperature variation. The flow correlated with a Poiseuille equation while friction factors were analyzed considering constant property ratio. However, it showed different correlation when considered variable property ratio. A numerical analysis on friction factor for single serpentine MICHX did not follow the Poiseulle law for both cases of constant property ratio and variable property ratio. The analysis showed significantly higher fRe values. The friction factor demonstrated strong dependency on log mean temperature difference, Reynolds number, and capacity rate ratio as expected. Introduction Channel size is very important in light of heat transfer and fluid flow characterization. Heat transfer devices with narrow channels are now the prime object of the researchers due to its augmented heat transfer capabilities. A multiport channel with flat geometry and common manifold can ensure uniform mass and velocity distribution even for a viscous fluid. Dehghandokht et al [1, 2] made numerical and experimental investigation on such type of multiport double slabs serpentine heat exchanger. They investigated glycol-water mixture varying the Reynolds Number of 850 - 2200. The authors found enhanced heat transfer rate with such geometry. Some investigators made their study on pressure drops and heat transfer characterization in mini or microchannels [ 3, 4, 5, 6, 7, 8]. Quaiyum et al [9] investigated mass flow and heat transfer characteristics of ATF flowing through a slab geometry minichannel heat exchanger. They found strong dependency of mass flow rates on fluid viscosity. The flow rate remarkably reduces due to viscosity when pressure drops are kept constant. The authors established correlation among the heat and mass transfer parameters such as; heat transfer coefficient, Nusselt number, Reynolds number, effectiveness, and viscosity effect on fluid flow. Many authors made their investigations on examining friction factors and viscous effect due to temperature variations [ 3, 10, 11 , 12, 13]. Koo et al [ 14] made dimensional analysis using water, methanol, and isopropanol to investigate effects of viscous dissipation on friction factor and temperature changes in the flow. For a micro-conduit they demonstrated that viscous dissipation made good correlation with Reynolds number, Eckert number, Prandtl number, conduit hydraulic diameter, and the channel aspect ratio. Khan, M.G. [ 15] made an intensive investigation on Heat Transfer, Pressure Drop, and friction factor characteristics. The author used Water and Glycol-Water Mixture in Multi-Port Serpentine Microchannel Slab Heat Exchangers and found strong correlation with Reynolds number. Wang et al [16] studied highly viscous fluid; water and lubricant flowing in minichannels to establish frictional characteristics. They found negligible influence of viscosity on friction factors for a hydraulic diameter greater than1.0 mm. Rudnick [17] while examining physical and chemical properties of ATF, the author assessed that at different temperatures the Viscosity Index Improver (VII) helps significantly in stabilizing properties of the viscous fluids. The VII is the long-chain polymers that get extended at higher tempe