A mixer employing a planar flip chip GaAs Schottky diode has been designed and tested at 24GHz. Using a planar diode eliminates the disadvantages of mechanical instability and large parasitical parameters associated with conventional whisker contacted diodes. The realized mixer provides conversion loss lower than 10dB and LO -to -RF isolation higher than 35dB. It is specially suitable for applications in automotive electronic system because of its simple structure and low cost.