The purpose of this study was to identify the key factors affecting the tax avoidance process in private companies. The present study is in the category of basic research in terms of purpose and in terms of nature and method, is a descriptive and case study with private companies in Iran. The statistical population of this research is experts familiar with the topic, which includes professors and experts in the field of taxation. A sample number of 20 experts answered the questions through purposeful judgmental sampling. In order to present the model and identify the key factors, the interpretive structural modelling approach with the help of Matlab software has been used as a qualitative approach. In the quantitative part, to weight and prioritize the factors, the multi-criteria decision approach has been used with the help of a network analysis process and Super Decision software. The results of interpretive modelling showed that the factors of financial leverage, company size, product market competition and financial constraints that are adjacent to the strategic red line, in addition to being strategic research variables, are classified as linked variables. The findings of the network analysis process also showed that the V10 criterion, which is "social trust", is the most important and therefore the most important sub-criterion in presenting the tax avoidance model. The V6 benchmark, which is "financial constraints", is then the second priority. Factors of competition in the product market and corporate social responsibility are ranked third and fourth.