The entry of the Czech Republic into the European Union resulted in many changes of law rules and regulations. These changes also affected a legislative amendment of the Accounting. The process of accounting reform is very closely related to the provision of reliable, comparable and comprehensible accounting information. The application of international accounting standards to national legislation and the creation of national accounting standards has become the basic trend in accounting. Changes were made by amendments to the Act on Accounting, which were followed by the issue of implementing regulations to the Act on Accounting and the Czech Accounting Standards in the past. Other changes to accounting legislation occurred in 2016, where the major changes included the valuation adjustment of inventory created by its own activities, making entries of transactions be connected with changes in inventories and their reporting. The aim of the paper is to evaluate the effect of changes in accounting of the unrealized production on the performance of companies in the manufacturing sector. Using the comparison of past and present methods of accounting for unrealized production will be an evaluation of observed changes and their impact on the profit and loss account and the selected financial indicators.