Xylanase significantly reduces the kappa number but almost not modifies brightness and viscosity in studied pulps. On average, xylanase decreases the kappa number of chemical pulps 25 %. This intense effect on the kappa number can be explained by enzymatic hydrolysis of hemicellulose. We proved that xylanase treatment on P0 pulp (first chlorine dioxide step) yield a kappa number reduction higher than obtained by chemical PO and D1 steps together. On the other hand, thermogravimetric analysis shows that xylanase treatment has influence on the thermal degradation path of pulps. As general rule, control treatment (with TrisHCl buffer but without enzyme) leads broads cellulose degradation peak, shown that chlorhydrate of tris(hydroxymethyl) aminoethane adsorb on cellulose surface increasing paracrystalline cellulose content, that is, not-ordered surface crystalline cellulose. When TrisHCl and xylanase are used together, adsorption is less important. In any case, in xylanase-treated pulps cellulose thermally degrades in broader temperature interval than in initial pulps