The current stablizer, reported in this paper, is part of a β-ray spectrometer. Principally, it can also be used in other apparatus requiring stablized current. The stablizer can supply a maximum current of 100 A with a degree of stablity of 0.02% in ten minutes.We adopt the principle of negative feed back sevo-control in designing the stablizer. Firstly, the error signal of the DC current passing through a standard resistance is amplified. In order to avoid the drift effect of the usual DC amplifier, we use a bridge type balanced modulator to convert DC signal into AC signal. After AC amplification, the signal is demodulated by a phase sensitive demodulator. The amplification required is about 2×l04. Concerning the amplifier, the following problems are considered in detail: to keep the output of the modulator in zero potential while there is no input signal, the effect of the carrier component of the modulator, and the distortion of the modulated signal after amplification. After power amplification, the amplified signal is used to control the field supply of the DC generator. As there are capacitance and inductance in the system, the transmission of the signal in the system has time delay. In closed system, there is the possibility of instability of the nature of low frequency oscillation, and it is necessary to insert proper network in the system to suppress the oscillation. Finally, the detailed arrangements of the different components of the stablizer, its characteristics, and result of its operation are described.