AbstractSpruce chemithermomechanical pulp was treated with a laccase mediator system (LMS) and alkaline H2O2, and the whiteness and dye removal index of the long fibres fraction and fines fraction were compared. The long fibres fraction and fines fraction were treated separately with a LMS, and their whitenesses and strengths were tested. The results indicated that the lignin and extractives contents of the fines fraction were higher than that of the long fibres. Because of the strong adsorption capacity and higher reactive efficiency, the optimal laccase dosage of the fines fraction was lower than that of the long fibres. In the process of treatment using a LMS and alkaline H2O2, at the same laccase dosage, separate treatment of the long fibres fraction and fines fraction could improve pulp whiteness and pulp strength.