Highly compensated Si was acquired by diffiusing Mn-metal into p-type silicon crystal of 5￡2 ? cm at high temperature. The relation between resistivity and temperature (temperature increases from 77 K to 300 K) was investigated with samples of 5.84 X 104fi ? cm, at room temperature. The results show that when the samples are in darkness, it behaves as common semiconductors; when the samples exposed to light, it behaves quite differently. That may caused by the fact that the material is sensitive to light and doped impurity of Mn lies on deep level.