A water/Oil type emulsified fuel has been demonstrated to decrease the harmful exhaust emissions and increase a thermal efficiency of internal combustion engine experimentally(1)*. However, an emulsified fuel is not currently used for internal combustion engines.When we try to use an emulsified fuel for the internal combustion engines, especially for a compression ignition engine, we should clear the physical properties of emulsified fuel. Because the viscosity of the emulsified fuel affects the spray characteristics and following combustion characteristics.It is commonly recognized that the viscosity of emulsified fuel is much higher than that of the base fuels. However, there are no data which show the viscosity increase of emulsified fuel.We proposed an newly introduced specific surface area Sp/Se to estimate the viscosity of the emulsified fuel. Se is the surface area of emulsified fuel which is assumed to be spherical, and Sp is the sum of the surface area of distributed water droplets (which also assumed to be spherical) in the emulsified fuel.The more the water content increases, the more the value of Sp/Se increases. The relative viscosity of the emulsified fuel is proportional to the value of Sp/Se.Furthermore, we showed that the emulsified fuel contains air bubbles which change the bulk modulus of elasticity. Therefore, the above mentioned physical properties of emulsified fuel affect the engine performance especially under transient operation. A pre-chamber type compression ignition engine was operated by an emulsified fuel under transient conditions to demonstrate the effect of above mentioned physical properties on the engine performance.