A high precision CMOS bandgap reference(BGR)voltage sourcce is described in this paper.With cascode current mirror and self-bias technique,through inverse feedback of OPAMP,it achieves current I_(PTAT) with positive temperature coefficient and current I_(CTAT) with negtive temperature coefficient respectively.Through current's subtraction,two different compensation current can be achieved in the whole temperature range,and then it can accomplish the piecewise compensation for the BGR.Therefore,we get the bandgap reference voltage with very small temperature coefficient.