In order to improve market rules in the energy sector of the European Union, it is obligatory to remove supplier barriers, harmonize tax and price policies and measures with respect to standards, while respecting environmental and safety regulations. The goal is to create a fully operational market, which guarantees everyone a fair access to the market, a high level of consumer protection and a satisfactory level of connectivity and capacity for production, transmission/transport and distribution of energy. The liberalization of the energy market boosts the expansion of market-driven business, whose main characteristic is competition. Notwithstanding the achieved results in the member states of the European Union, it is still necessary that entitled regulatory authorities (NRAs) monitor the functioning of national energy markets. When carrying out the analysis, the NRAs use different measures to determine the concentration of observed markets. In general, high market concentration suggests the possibility of monopolistic action in such a market that needs to be prevented, because the dominant company may increase prices above the competitive price without fear of profit loss. In such a situation, end users lose the most due to higher prices. Furthermore, market competition represents a simple and efficient way to guarantee good products, services and competitive prices to customers. In this thesis, the basic characteristics of the Croatian gas market reform process are presented with an emphasis on the liberalization process. The legal framework related to the antimonopoly activities of the entitled bodies has been analyzed. The role of the competition authorities and bodies entitled for regulation of energy activities as well as the measures of market concentration are examined. Market concentration indices determine market concentration and give information in which direction such a market should be developed. High market concentration points out the need to take regulatory or legislative measures to introduce new market participants. On the contrary, there is an open market with efficient competitiveness in the case of a low market concentration. Furthermore, in order to improve the efficiency of the business of the companies that carry out distribution and supply of gas, the Ramsey's rule is applied for the example of consolidation of market participants by entering into new markets or expanding their activities into new distribution or supply areas.