Fibre contained in vegetal raw materials can change during various technological treatments including cooking. The objective of this study was to determine the impact of thermal treatment method, i.e. cooking in water and steaming, on the content of dietary fibre in brassica vegetables. The material analyzed included: white cabbage of the Cilion cultivar, red cabbage of the Lektro cultivar, and Savoy cabbage of the Fiona cultivar. In those raw materials, determined were the contents of total dietary fibre (TDF) and of soluble (SDF) and insoluble (IDF) fractions using an Asp’a method. Based on the obtained results, it was found that the thermal treatment of brassica vegetables impacted the content of dietary fibre therein. Vegetables cooked in water were characterized by a significantly higher content of dietary fibre compared to steamed vegetables.