Changes in water content and viscosity for A- and C-heavy oil during emulsion formation are experimentally examined. The A-heavy oil hardly forms a W/O emulsion. For C-heavy oil with sea water, the rate of emulsion formation and water content for the same shaking time are lower than those of C-heavy oil with distilled water. Regardless of the kind of water phase, the relationship between water content and viscosity of the emulsion of C-heavy oil is well expressed by the Mooney equation.