The invention relates to a process for manufacturing liquid packaging board, in which process a board pulp is treated with a percarboxylic acid by adding to the pulp percarboxylic acid in an amount of 0.5-5 kg/ton of dry pulp, indicated as a 100 percent percarboxylic acid, whereafter, or simultaneously with the percarboxylic acid treatment, sizing is carried out with a combination of a resin size and a neutral size, which is followed by board forming. The manufactured board has good resistance to liquid penetration.