A motor driven air compressor with a fixing device for the motor, the motor driven air compressor being received in an accommodation box and comprising: a body includes multiple positioning orifices which are a first positioning orifice and a second positioning orifice; a cylinder connected on the body and communicating with an air storage holder; a motor fixed on the body, a small gear being received in the first positioning orifice of the body and being connected on an end of the motor, and a connection seat of the motor being accommodated in the first orifice; a transmission mechanism actuating a piston to move in the cylinder reciprocately so as to produce compressed airs, wherein at least one retainer is configured to fix the body and the motor, wherein a first end of the at least one retainer is engaged on the body, and a second end of the at least one retainer is engaged on a casing of the motor so that the motor is fixed on the body without using any screws, wherein the motor includes two dissipation holes symmetrically formed on two sides of the casing, characterized in that the first end of the at least one retainer is an open segment, and the second end of the at least one retainer is a close segment, wherein the open segment of the at least one retainer has a first bent engagement portion, and the close segment of the at least one retainer has a second bent engagement portion, and wherein, when the transmission mechanism is connected on the body and the cylinder is connected with an upper portion of the body opposite to the transmission mechanism, a first external fence and a second external fence of the body having two receiving grooves and two third positioning orifices which are all defined on the first external fence and the second external fence, the at least one retainer is received in the two receiving grooves of the body, the first bent engagement portion of the open segment of the at least one retainer is engaged with the two third positioning orifices of the body, and the second bent engagement portion of the close segment of the at least one retainer is engaged with the two dissipation holes of the casing of the motor. a body includes multiple positioning orifices which are a first positioning orifice and a second positioning orifice; a cylinder connected on the body and communicating with an air storage holder; a motor fixed on the body, a small gear being received in the first positioning orifice of the body and being connected on an end of the motor, and a connection seat of the motor being accommodated in the first orifice; a transmission mechanism actuating a piston to move in the cylinder reciprocately so as to produce compressed airs, wherein at least one retainer is configured to fix the body and the motor, wherein a first end of the at least one retainer is engaged on the body, and a second end of the at least one retainer is engaged on a casing of the motor so that the motor is fixed on the body without using any screws, wherein the motor includes two dissipation holes symmetrically formed on two sides of the casing, characterized in that the first end of the at least one retainer is an open segment, and the second end of the at least one retainer is a close segment, wherein the open segment of the at least one retainer has a first bent engagement portion, and the close segment of the at least one retainer has a second bent engagement portion, and wherein, when the transmission mechanism is connected on the body and the cylinder is connected with an upper portion of the body opposite to the transmission mechanism, a first external fence and a second external fence of the body having two receiving grooves and two third positioning orifices which are all defined on the first external fence and the second external fence, the at least one retainer is received in the two receiving grooves of the body, the first bent engagement portion of the open segment of the at least one retainer is engaged with the two third positioning orifices of the body, and the second bent engagement portion of the close segment of the at least one retainer is engaged with the two dissipation holes of the casing of the motor.