The aim of our diploma thesis "Leakage of a dangerous substance in a traffic accident" was to simulate a traffic accident and model the leakage of a selected dangerous substance during the transport on the road. For model situation we used program ArcGIS. Thanks to the results of model situation was possible to find out the consequences and preparedness of Rožňava city. At the end we described how work the integrated rescue system during the traffic accident. The thesis has two main parts. The first part is focused on explanation of the main concepts that are in thesis. We focused on legislation, problematic at home and abroad, we defined dangerous substances, proceeding of transporting the dangerous substances, classification, packaging and labeling. We also explained the term traffic accident, we presented the instructions during the traffic accident. We also presented some traffic accidents which happened in Slovakia and abroad. In the second part, we modeled the situation during the leakage of a selected dangerous substance. The dangerous substance we chose was - heavy heating oil, which was transported by cistern. On the road of first class, we modeled a traffic accident. The heavy heating oil leaked out on the road and into the aquatic environment. Subsequently, we described how work the integrated rescue system during the traffic accident from beginning till the elimination of traffic accident.