Abstract - This paper pertains to the analysis of power consumption of the C - MOS crystal oscillator with a high frequency crystal unit. It is assumed that the total power consumption of the C - MOS crystal oscillator consists of two parts ; I ) the part which is determined by the current through N type and P type FET and 2) the part required to charge the load capacitance of the C-MOS amplifier. The relation between these power consumption and the value of each element in the oscillator ciruit has been analysed theoretically and experimentally. It is found that the power consumption for the charging of the output capacitance Cd of the C - MOS amplifier is the dominant part of the total power consumptioon. The optimum relation between gate capacitance Cg and output capacitance Cd is found which gives the minimum value of the average current from power source.