Block copolymers of poly(vinyl alcohol) (PVA) and polyacrylamide (PAAm) were investigated as paper additives. The mixture of both homopolymers in aqueous solution caused phase separation, which indicated incompatibility. The block copolymers were soluble in water without macrophase separation. In the case of surface sizing to paper, PVA-b-PAAm as well as PAAm afforded stiffness to paper, whereas PVA could not. In the case of internal sizing, the block copolymers with anionically modified PAAm revealed excellent improvement in paper properties, whereas PAAm made the tearing strength worse. Brittle polymers such as PAAm appear to afford stiffness to paper, but also make the paper brittle. PVA that is a tough polymer due to high crystallinity and low glass transition temperature can afford toughness to paper, at the expense of stiffness. The block copolymers seem to retain the desirable properties of each simultaneously.