The research problem of this paper is the harmonization of the tax systems of the member states of the European Union, which is an important factor in the establishment and functioning of the single market. Due to the inconsistency of the tax system, the fiscal policy of the European Union is not uniform. The goal of the work was to perform an analysis and determine the degree of compliance of both indirect taxes (VAT, excise duties or excise taxes) and direct taxes (income tax, profit tax). As a result of the research, it was determined that each member state has the right to keep its tax system, even to introduce tax forms or with the obligation that some of their parts will be harmonized with the decisions of the European Union bodies. The European Union has adopted several directives or guidelines on tax harmonization, and a certain minimum harmonization of tax systems has been achieved. Partial harmonization has been achieved with direct taxes, while the furthest has been achieved with the harmonization of indirect taxes, i.e. excise duties and VAT. The basis for harmonizing the taxation of indirect taxes is defined in Articles 90 to 93 of the Treaty on European Union. The Sixth Directive was the basic document of the European Union and the legal basis for the harmonization of VAT. An agreement was reached whereby the members of the European Union apply the standard rate of VAT, which cannot be lower than 15%. The Common Excise Tax System was introduced in 1993, which specifically applied to alcoholic products, tobacco products and energy products (coal, coconut, oil derivatives, electricity, natural gas). Considerable progress has been achieved in the harmonization of excise duties and in the movement of goods subject to excise duties.