The conventional TEMPO/NaBr/NaOCl system for oxidation of cellulose to prepare nanocellulose materials has some shortcomings in terms of controlling side reactions and clogging in washing/filtration process. A new TEMPO/CaBr2/Ca(OCl)2 system was then developed to oxidize a hardwood bleached kraft pulp (HBKP) in water at pH 10 (TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl radical). An aqueous Ca(OH)2 solution was used to continuously control the reaction mixture at pH 10. After oxidation, the reaction mixture containing the oxidized products and chemicals was directly filtered on a 40-μm-mesh nylon filter and the water-insoluble oxidized products on the filter were washed with water without any clogging. The carboxy content increased to 1.5 mmol/g and the mass recovery ratio decreased to 87.7% as the amount of Ca(OCl)2 was increased to 10.0 mmol/g-HBKP. The oxidized products contained calcium ions but almost no chloride ions, indicating that they comprised almost pure –(COO)2Ca groups. The ready filtration and washing of the oxidized products was probably owing to the low degree of dissociation of the –(COO)2Ca groups in water. The X-ray powder diffraction (XRD) and solid-state carbon 13 nuclear magnetic resonance (13C-NMR) analyses revealed that the crystallinities and crystal widths of the original cellulose I structure were mostly retained in the oxidized products. However, size-exclusion chromatography and viscosity analyses revealed that substantial depolymerization occurred on the cellulose and oxidized cellulose molecules in the products, as in TEMPO/NaBr/NaOCl-oxidized products.Graphical