Aim/purpose - The purpose of this article is a presentation of the principles of tax law equality with the aim of establishing the significance of these rules for the system of direct taxation in Poland. Only forms of taxation have been selected to conduct this analysis because they offer the most transparent and variegated picture of the real tax burden. Design/methodology/approach - The article follows the method of economic comparative analysis and offers a review of available literature on the subject. Findings - This analysis proves that the choice of different types of income taxation for natural persons conducting business activity has influence on tax burden. Research implications/limitations - The system of direct taxation in Poland has problems with equality rules because of different tax rates and tax returns which are available in Polish law. Originality/value/contribution - This article presents the problem of tax law equality in the context of economic theories developed by selected economists.