A method for catalytic oxidation of cellulose using a heterocyclic nitroxyl radical as catalyst, main oxidant acting as oxygen source, and an activator of the heterocyclic nitroxyl radical, comprises—a preliminary activation step of the heterocyclic nitroxyl radical with hypochlorite to activated catalyst, and after the preliminary activation step, a cellulose oxidation step using the activated catalyst and hypochlorite as the main oxidant to oxidize cellulose. In the cellulose oxidation step, fibrous starting material is subjected to the oxidation to oxidize the cellulose, whereafter the oxidized fibrous raw material is disintegrated to nanofibrillar cellulose (NFC).