Dialkylnaphthalene is oxidized to naphthalenedicarboxylic acid in a lower aliphatic carboxylic acid solvent in the presence of a catalyst comprising a cobalt compound, a manganese compound and a bromine compound having an atomic ratio of manganese to cobalt of 0.03 to 0.5. The catalyst is supplied to the oxidation reaction zone so that the total amount of cobalt and manganese is 0.025 to 0.1 gram atom based on 1 gram mol of the dialkylnaphthalene. An oxidation product slurry is subjected to solid-liquid separation when the concentration of the naphthalenedicarboxylic acid in the slurry is 8 to 30% by weight. The process prevents the by-production of benzotricarboxylic acid such as trimellitic acid, thereby drastically reducing the incorporation of heavy metal complexes with trimellitic acid into naphthalenedicarboxylic acid crystals. The process optionally includes the addition of a polymer flocculant, this forming crystal aggregates with a large particle size and facilitating the separation of the crystals from the mother liquor.