The purpose of the presented research work was to find out COD fractions in raw wastewater and during treatment with constructed wetland (CW) system. The tests were performed in CW system with average flow of about 4 m³/day. In raw wastewater the highest concentration of fraction in organic suspended solid easy biodegradable (X_S – 250 mg O₂/l) was observed. The same situation was with dissolved organic easy biodegradable matter (S_S – 250 mg O₂/l). Lower quantity of non-biodegradable fractions dissolved and in suspended solids were observed (S_I – 27 mg O₂/l, X_I – 83.33 mg O₂/l). More than 80% of total COD was as biodegradable fractions (S_S + X_S). After treatment with CW system the highest concentration was observed for dissolved non-biodegradable fraction (S_I – 27 mg O₂/l). Lower concentration was for biodegradable fraction in suspended solid (X_S – 3.33 mg O₂/l). More than 89% of total COD was in biological non-biodegradable fraction (S_I + X_I).