The master thesis "Optimization of user experience of web stores through 10 Nielsen usability heuristics" investigates the importance of usability as an integral part of user experience (UX) for IT web stores. The goal of the work is to show the key role of usability when designing online stores in order to provide users with the easiest possible navigation and purchase of products, as well as ensure complete satisfaction and to create a desire to repeat the process in the future. Also, the aim of the thesis is to provide concrete guidelines for improving the user experience on selected websites. In order to achieve the desired goals of this thesis, reconnaissance research was carried out through the concept of heuristic analysis. Heuristic analysis is used by user experience experts in a way that the website is analyzed based on valid standards and guidelines, and the final rating is the result of the judgment of the researcher who conducted the evaluation. For this method, analysis through Nielsen's 10 usability heuristics was chosen. Each heuristic forms a whole with a series of questions that go into its details, giving a clearer overview of the analyzed elements. IT web stores were selected as a sample due to the complexity of the products that are sold and the high level of information and trust that must be achieved before making a purchase decision. After the analysis of both sites, Newegg and PC shop, the results for each heuristic and the overall rating according to the importance and level of adherence to the criteria were calculated. Newegg adhered to most of the questions within the heuristics and followed the best practices in the industry and achieved a high score of 96.13%, while PC shop showed minimal effort in usability optimization, achieving a score of 54.51%. Based on the analysis, a list of main problems and usability solutions necessary to increase the overall rating was made.