Abstract: An apparatus for preparing a water/diesel oil micro-emulsion comprises a mixing device operatively connected to a mixing tank. The mixing device comprises a cone shaped septum placed in a duct, coaxial with respect to a main direction and tapering towards an outlet nozzle. The cone shaped septum is provided with a plurality of holes made through its conical wall. A plurality of lamellae is arranged in at least a portion of the duct placed downstream of the cone shaped septum, divides the portion in a plurality of small chambers and is provided with through holes. The holes and the small chambers delimit a labyrinth passageway for the liquid flowing through the duct towards the outlet nozzle.