A system for evaluating the DGA results of power transformers 35-220 kV according to the criteria for permissible and maximum permissible values of gas concentrations and their growth rates is given in the article. By means of an varince analysis, design features of transformers have been revealed that have the greatest influence on the rate of formation of gases. The results of the variance analysis were then taken into account when constructing the system under consideration. The criteria are calculated on the basis of an impressive array of DGA results accumulated in the database of the expert-diagnostic system for 25 years of its work. The algorithm for determining the required periodicity of DGA and necessary maintenance operations is given in the article.