A cooling fan comprising the following features, wherein the words top and lateral refer to a position of the cooling fan in which its rotation axis is vertical: a housing including a support portion (1 a), a cover portion (1 b) and a lateral wall portion, with the lateral wall portion disposed between the cover portion and the support portion, with the lateral wall portion, the support portion and the cover portion (1 b) defining a compartment, and with the lateral wall portion (1 c) having at least one lateral air inlet and at least one lateral air outlet penetrating through the lateral wall portion and communicating with the compartment; and a motor mounted inside the compartment of the housing and containing a stator and an impeller, with the impeller rotatably mounting to the stator, wherein the impeller further contains a hub with a top and a plurality of blades, with the hub containing a cylindrical wall, with the top facing the cover portion, with the top and the cover portion forming a lateral flow path in the compartment, and with each blade contained in the lateral flow path, wherein an annular plate radially extends from an upper part of an outer periphery of the cylindrical wall of the hub adjacent to the top of the hub, and wherein the blades are mounted on the annular plate and extend towards the cover portion, wherein the support portion forms a recess in communication with the compartment, with the stator of the motor and the hub of the impeller received in the recess, wherein the motor is an outer rotor motor, and the hub which is received in the recess is a rotor part of the motor, and wherein the recess has a radial extent from a central axis of the recess, wherein the annular plate has a radial extent from the central axis of the recess larger than the radial extent of the recess, wherein the cooling fan is characterized in that an upper surface of the annular plate faces the cover portion (1 b), the upper surface has a level difference (d) relative to the top of the hub in an axial direction of the housing, and the level difference (d) is smaller or equal to half of an axial length of the blades and larger than zero. a housing including a support portion (1 a), a cover portion (1 b) and a lateral wall portion, with the lateral wall portion disposed between the cover portion and the support portion, with the lateral wall portion, the support portion and the cover portion (1 b) defining a compartment, and with the lateral wall portion (1 c) having at least one lateral air inlet and at least one lateral air outlet penetrating through the lateral wall portion and communicating with the compartment; and a motor mounted inside the compartment of the housing and containing a stator and an impeller, with the impeller rotatably mounting to the stator, wherein the impeller further contains a hub with a top and a plurality of blades, with the hub containing a cylindrical wall, with the top facing the cover portion, with the top and the cover portion forming a lateral flow path in the compartment, and with each blade contained in the lateral flow path, wherein an annular plate radially extends from an upper part of an outer periphery of the cylindrical wall of the hub adjacent to the top of the hub, and wherein the blades are mounted on the annular plate and extend towards the cover portion, wherein the support portion forms a recess in communication with the compartment, with the stator of the motor and the hub of the impeller received in the recess, wherein the motor is an outer rotor motor, and the hub which is received in the recess is a rotor part of the motor, and wherein the recess has a radial extent from a central axis of the recess, wherein the annular plate has a radial extent from the central axis of the recess larger than the radial extent of the recess, wherein the cooling fan is characterized in that an upper surface of the annular plate faces the cover portion (1 b), the upper surface has a level difference (d) relative to the top of the hub in an axial direction of the housing, and the level difference (d) is smaller or equal to half of an axial length of the blades and larger than zero.