A pneumatic radial tire for heavy loads, having steel belt layers provided on the inner side of a tread, and a tread pattern having a plurality of divisional ribs separated by a plurality of main grooves extending in the circumferential direction of the tire, characterized in that at least the inner vertical surfaces of left and right outermost main grooves out of the main grooves are formed so as to extend zigzag in the circumferential direction of the tire, narrow grooves which extend continuously in the circumferential direction of the tire being provided at the shoulder side edge portions of ribs internally adjacent to the outermost main grooves, the width W.sub.4 of the narrow grooves being set to a level in the range of 0.5-1 mm, the axes of the narrow grooves which extend in the direction of the depth thereof being inclined in the downwardly inward direction with respect to a normal of the tread surface, the narrow grooves forming laterally separated rib bodies and narrow ribs the height of which is smaller than that of the rib bodies, a ratio of a maximum width W.sub.2 of the narrow ribs to a total width W.sub.1 of the ribs on the inner side of the outermost main grooves being set to a level satisfying the relation W.sub.2 /W.sub.1 .ltoreq.0.3, an angle .alpha..sub.1 between the side surface which faces the relative outermost main groove of a maximum width portion of each narrow rib and a normal of the tread surface being set smaller than an angle .alpha..sub.2 between the side surface which faces the relative outermost main groove of a minimum width portion of each narrow rib and the same normal.