Abstract: This paper presents effective evaluation of well log interpretation by fully utilizing well log data, based on basic principle and method of formation system. That is, using well log data and the results from formation test as a decision-making table of information system，from which the decision making regulation with uncertainty is taken out as its intelligent model in terms of theory of fuzzy set and rough set. Such a model has the characteristics of high non-linearization, high validity and high generalization in use, especially suitable for the evaluation of complex heterogeneous strata as a new tool to well log interpreters.

Key words: well log interpretation, model, fuzzy math, information processing, decision-making analysis