Abstract: A method of preparing a paper having a reinforced strength wherein a polymer latex is added to a paper stock in an amount of from 2.5 to 30% by weight. The polymer latex is prepared by neutralizing a seed polymer with an acid or a salt or by quaternizing it with a quaternizing agent. The surfaces of the emulsion grains are provided with a large number of cationic groups. The seed polymer is formed by adding a monomer represented by the following formula (I) or a mixture of the monomer (I) and copolymerizable ethylenic unsaturated monomer(s) (II) to a seed latex of a carboxyl-modified synthetic rubber latex or synthetic resin emulsion and polymerizing them: ##STR1## R.sub.1 represents H or CH.sub.3 ; R.sub.2 represents an alkylene group having from 2 to 5 carbon atoms;R.sub.3 and R.sub.4 each represents H or an alkyl group having from 1 to 5 carbon atoms;A represents ##STR2## and R.sub.1, R.sub.3 and R.sub.4 are so selected that the monomer is hardly soluble or insoluble in water.