Abstract: The methodology of estimating oil and gas resources has been widely investigated only in recent years. Now, the prospects of oil and gas resources have conspicuously become the most important research project. The methodology does not deal with concrete methods of estimation, but examines the valuation of the various methods of estimation under consideration from the overall viewpoint of searching for oil and gas resources. The contents of the research in methodology of estimation are discussed, and the core of such a matter is bow to establish an estimation system. The methodology concerning how to define an estimating problem, to divide estimated objects, to use various kinds of estimating method properly, and to improve progressively the estimation system is discussed. The evaluation method by specialists and the Monte Carlo method can play an important role in establishing an estimation system.