The changes taking place both in societies as well as in the world's economy, resulting from the increase of the international business and cultural exchange, caused the necessity of harmonization in the area of broadly defined business accounting. Financial statements are crucial in the context of making decisions on investment, development and strategies, as well as, for perception of the company by outside users of these reports. Striving at the solid presentation of information based on economic events registered by the company requires a good knowledge of the accounting rules. Accounting rules used in financial reporting play an important role in the evaluation of financial performance of a company. The problem of converting the financial statements prepared according to the Polish accounting law into the statements based on the principles of the International Financial Reporting Standards (IFRS) is widely described in the literature. However, there is a deficiency in studies presenting the effects of converting financial reports prepared according to the Polish accounting law into the statements based on the principles of the International Accounting Standards in the context of evaluation of the company's financial resources, assets, and income. The goal of this paper was to indicate the areas and extent of discrepancy of the financial report prepared according to the Polish balance law and that consistent with the International Financial Reporting Standards, as well as to evaluate the impact of the changes on the balance structure, income statement and basic financial ratios. The results of the study prove that the differences between the Polish balance law and the International Accounting Standards (IAS) concerning the presentation of financial reports had a significant impact on the evaluation of financial situation of the company examined and the usefulness of financial reports for the readers as well as the quality of decisions.