Internal audit is an independent function of a company whose purpose is to control the operations of a company. This paper describes the work of internal audit through the example of the company Tvrtko d.o.o. Based on the financial statements of Tvrtko d.o.o. the calculation of the financial indicators is presented and an analysis of the financial indicators has been shown. Furthermore, an analysis of the environment based on the company's business was developed. The analysis of the environment has defined the advantages and threats in the company's business. Internal audit, based on an analysis of the environment, points to the need to pay attention and the advantage of the company versus the competition. Below is an internal audit plan based on financial indicators and analysis of the environment. Five-year and annual plan and one-tier annual plan were prepared. At the end, the work of internal audit is presented at the reclamation department. In the example section of the complaint, it is presented how the internal audit finds the disadvantages and gives suggestions to the management of the company how to improve the department's business