Abstract: The formation of water-in-crude oil emulsion can be encountered in many stages such as drilling, transporting and processing of crude oil. To enhance and control these processes, it is necessary to understand the emulsion mechanisms. In this study, two types of Malaysian crude oil namely; heavy crude oil and light-heavy blended crude oil (40-60 vol%) were characterized physically to use as the oil phase. Cocaamide DEA was used as a natural surfactant. The stability of water-in-crude oil emulsion were investigated at different water volume fractions (50 and 20%) with four volume fractions of Cocamide DEA (0.2 , 0.5, 1 and 1.5%). These emulsions were tested for relative rates of water separation and for rheology studies to demonstrate the viscosity behavior on the emulsion stability. Brookfield Viscometer was used to study the effects of shear rate, temperature, rotation speed (rpm) and water content on the viscosity at varying temperature (30-90°C) and rotation speed (50-250 rpm). Results showed that higher concentration of Cocamide DEA and lower water volume fraction were effective in stabilizing the water-in-crude oil emulsion at room temperature. In addition, the viscosity of the emulsion stabilized by Cocamide DEA showed non-Newtonian behavior.