This paper describes a high stability miniature size OCXO using a SC-cut crystal resonator with simultaneous oscillations as the method of taking two signals from one crystal, compared with the Twin-Crystal technology [2] which had the high accuracy temperature sensor by the two crystal units. This 9 × 7 mm size OCXO has satisfied around +/- 1 to 2 ppb in a range of ambient temperature from −40 to +85 °C by the ideal temperature sensor.