Carbon/oxygen spectral log is a nuclear logging method for detectingchemical indicator elements in strata. Based on surveyed C/Oratios and Si/Caratios, we can determine water saturation and water cut of strata in casedwell.Since the beginning of 1980’s, carbon/oxygen log has been performed in suchoil fields of China as Daqing, Shenli, Liaohe and Dagang with satisfactorygeologic results achieved. In this paper the fundamentals and rield applicationexamples of the carbon/oxygen spectral logging method for interpretation ofhydrocarbon and water zone are discussed in detail.