There are provided a pneumatic radial tire of heavy load vehicles, in which durability of a belt is improved.A pneumatic radial tire for heavy load vehicles comprises a pair of bead portions in each of which a bead core is embedded, a radial carcass ply 1 extending from one bead portion to the other bead portion and turned around the bead core from an inner side to an outer side of the bead core in a width direction of the tire and at least six belt layers 2 disposed on an outer side of this radial carcass ply 1 in a radial direction of the tire, wherein the following relations are satisfied: W56>W34>W12, where W12 is a maximum width of first and second belts 2a f the belt layers 2, W34 is a maximum width of third and forth belts 2b of the belt layers 2 and W56 is a maximum width of fifth and sixth belts 2c of the belt layers 2; and 1>D12/D34>0.6, where D12 is a cord diameter of the first and second belts and D34 is a cord diameter of the third and forth belts. In addition, a steel cord of each belt layer has a double twist structure in which a plurality of steel filaments are twisted to form a cable and two to ten cables are further twisted.