A miniature 40 MHz OCXO with low phase noise is designed according to the use needs. The oscillator is designed with low noise LDO as power processing，SCcut crystal resonator and Bmode restrained network. The phase noise is improved while the size is reduced. The temperature control system adopts the direct heating circuit, the proportional integral algorithm, and the active load as the heating element to improve the temperature control accuracy of the product and reduce the influence of the ambient temperature change on the oscillator. The oscillator is stable and reliable, operating at only 5 V. The obsolute value of frequencytemperature characteristic is not more than 0.02×10-6（-30～+70 ℃）, and the phase noise is not more than -160 dBc/Hz@1 kHz. The power consumption is only 0.9 W and the size is only 20 mm×12 mm×10 mm.