The article is devoted to a comprehensive analysis of the problem of harmonization of tax law of the European Union and its impact on the national legal order of member states on the example of the Republic of Poland with a comparison with the legal system of the Republic of Kazakhstan. The relevance of the study is due to the deepening of integration processes, the strengthening of supranational regulation in the field of corporate taxation and the formation of minimum generally binding standards for countering the erosion of the tax base. The paper examines the regulatory grounds for harmonization enshrined in the law of the European Union, the mechanisms for implementing directives in Polish law, as well as the role of the judicial practice of the Court of Justice of the European Union in ensuring a uniform interpretation of tax standards.

The methodological basis of the study is comparative legal, formal legal and systemic methods. The empirical base includes the jurisprudence of Polish administrative courts, official reports of financial authorities, Eurostat statistics and materials of international organizations for 2020-2025.

As a result, it has been established that harmonization in the European Union has evolved from a technical convergence of indirect taxes to the formation of a multi-level regulatory model affecting the basis of corporate taxation. It has been proven that this process does not eliminate the tax sovereignty of states, but transforms it into a form of joint implementation of competence. Comparison with the model of coordination within the framework of Eurasian integration made it possible to identify differences in the depth and binding legal rapprochement.

A conclusion was formulated on the formation of a stable system of multi-level tax regulation, combining national autonomy and supranational mechanisms for ensuring financial stability and legal certainty.