This paper analyses the design of low phase noise oven controlled crystal oscillator (OCXO). Especially, the influence of the loaded Q value and circuit topology on the phase noise is introduced with emphasis. The oscillator circuit, low noise buffer amplifier circuit and regulator circuit are analyzed in detail. Then a miniature ultra low phase noise OCXO has been designed based on the modified Butlertype circuit. The developed miniature ultra low phase noise OCXO with 100 MHz has the size of 20 mm×20 mm×10 mm and phase noise of -168 dBc/Hz@1 kHz.Test results proved that the design of the proposed OCXO is successful.