Apparatus for preparing a water/diesel oil micro-emulsion, comprising: at least one diesel oil feeding unit; at least one emulsifying composition feeding unit; at least one water feeding unit; at least one mixing tank in fluid communication with the diesel oil feeding unit, with the emulsifying composition feeding unit and with the water feeding unit; a device for mixing water and diesel oil operatively connected to said mixing tank; characterized in that the device for mixing water and diesel oil comprises: at least one duct for a flow of liquid, said duct extending along a main direction (X-X) and presenting an inlet opening and an outlet nozzle; a cone shaped septum placed in the duct, coaxial with respect to the main direction (X-X) and tapering towards the outlet nozzle, said cone shaped septum being provided with a plurality of holes made through its conical wall; a plurality of lamellae (76, 80) arranged in at least a portion of the duct placed downstream of the cone shaped septum, said plurality of lamellae (76, 80) dividing said portion in a plurality of small chambers (77, 82), wherein said lamellae (76, 80) are provided with through holes (78, 81); said holes (78, 81) and said small chambers (77, 82) delimiting a labyrinth passageway for the liquid flowing through the duct towards the outlet nozzle. at least one diesel oil feeding unit; at least one emulsifying composition feeding unit; at least one water feeding unit; at least one mixing tank in fluid communication with the diesel oil feeding unit, with the emulsifying composition feeding unit and with the water feeding unit; a device for mixing water and diesel oil operatively connected to said mixing tank; characterized in that the device for mixing water and diesel oil comprises: at least one duct for a flow of liquid, said duct extending along a main direction (X-X) and presenting an inlet opening and an outlet nozzle; a cone shaped septum placed in the duct, coaxial with respect to the main direction (X-X) and tapering towards the outlet nozzle, said cone shaped septum being provided with a plurality of holes made through its conical wall; a plurality of lamellae (76, 80) arranged in at least a portion of the duct placed downstream of the cone shaped septum, said plurality of lamellae (76, 80) dividing said portion in a plurality of small chambers (77, 82), wherein said lamellae (76, 80) are provided with through holes (78, 81); said holes (78, 81) and said small chambers (77, 82) delimiting a labyrinth passageway for the liquid flowing through the duct towards the outlet nozzle. at least one duct for a flow of liquid, said duct extending along a main direction (X-X) and presenting an inlet opening and an outlet nozzle; a cone shaped septum placed in the duct, coaxial with respect to the main direction (X-X) and tapering towards the outlet nozzle, said cone shaped septum being provided with a plurality of holes made through its conical wall; a plurality of lamellae (76, 80) arranged in at least a portion of the duct placed downstream of the cone shaped septum, said plurality of lamellae (76, 80) dividing said portion in a plurality of small chambers (77, 82), wherein said lamellae (76, 80) are provided with through holes (78, 81); said holes (78, 81) and said small chambers (77, 82) delimiting a labyrinth passageway for the liquid flowing through the duct towards the outlet nozzle.