The subject of research of the master's thesis are the systems of corporate income tax of the two countries, members of the European union, with significantly different states of economy, but the same corporate tax rates on profit. The first will be analyzed the tax system of the Republic of Croatia, which is characterized by only two decades of independence and a number of adjustments and experimentations in the tax system due to the transition to a capitalist system and pre-accession agreements with the European union, while on the other hand we have the UK, which is among the first countries who began to develop their own tax systems and which has a long tradition of the corporate income tax application. The intention is that with simplified defining, historic features, reasons and economic validity along with the ways the corporate income tax is applied, to make the basis for a constructive comparison of the corporate income tax systems between the Republic of Croatia and the United Kingdom. Through the separate identification of taxpayers, ways of calculating the tax base, tax rates and the tax period, and the recognition of allowances, incentives and tax exemptions, the corporate income tax system of the Republic of Croatia and the United Kingdom will be investigated and compared in details, where comparative analysis will be made, along with advantages and disadvantages of the one system over the other. On the practical example of a small company that obtains business in both countries, it will be discovered which country may constitute a better and simpler form of corporate income tax and thus contribute to the more successful development of the economic environment and more efficient filling of the state budget. The analysis of these two systems will lead us to conclusion of the importance of corporate income tax in the economic development in relation to other factors.