The microstructure formation and phase selection characteristics of Co-Cu peritectic alloys during rapid solidification are investigated experimentally. The re lationships between such physical factors as cooling rate, microstructure morpho logy, crystal orientation, and alloy resistivity are further analyzed. The exper imental results show that rapid solidification makes the Co solubility in (Cu) p hase extend up to 20%Co. If Cu concentration is more than 80%Cu, the peritectic transformation L+αCo→(Cu) is suppressed, and the (Cu) phase can directly preci pitate from the undercooled alloy melt. When the Cu concentration is in the rang e of 40%-70%Cu, the liquid phase separation of the alloys is also suppressed, and the microstructure in the direction of ribbon thickness consists of two crysta l zones. In the fine crystal zone, the αCo and (Cu) phases nucleate and grow co mpetitively, the tiny equiaxed (Cu) dendrites are distributed in the αCo matrix homogeneously, whereas in the coarse crystal zone the αCo is the leading phase , where Cu-rich phase exists in the grain boundary of αCo dendrites. With the increase of cooling rate, the microstructure is refined and the amount of grain boundary increases, resulting in the rise of alloy resistivity. If the grain bou ndary reflection coefficient r＝0996—0999, the electrical resistivity o f Co-Cu peritectic alloys can be predicted theoretically.