Abstract: In order to realize an computerired automatic electrofacies analysis for determining the lithology of beds penetrated by a well, a combination of in-zone variance method with cluster aralysis technique is used for determining the zone boundaries by well logging curves. The types of electrofacies of standard sample beds from key well are classified by a combination of principal component and cluster analysis. The discriminant models of each electrofacies are established by Bayes'step-by-step discriminant analysis. An electrofacies-lithology model of oil field is established by detailed correlation of electrofacies with core data. Finally a continualy bed-by-bed electrofacies analysis is carried out for key well and most wells without coring. Consequently a continuoasly and complete lithologic profile can be obtained. The electrofacies analysis software based on the above mentioned methods has been developed. The well logging data of several wells located in Shen Li, Da Gang and Hua Bei oil fields have been processed by this software. The results are coinsided to be very well with the data of cores and drilling cuttings. It has been demonstrafed that the suggested methods are sucessful, the software are applicable, and the results of automatic electrofacies determination are satisfactory.