A pneumatic radial tire has a thread surface extending along a specific curvature plane which includes a first arc with the radius R1 having a center on the tire's equatorial plane and passing through the tire's equatorial point, a second arc with the radius R2 having a center on the tire's equatorial plane and intersecting with the first arc at an intersection distant from the tire's equatorial plane by 0.2 to 0.25 times the tire width SW, and a third arc with the radius R3 passing through a ground contact outer edge point of the ground contact surface, when a standard load is applied, and a belt intermediate height point on the tread surface. In the specific curvature plane, the curvature radius ratio R2/R3 is set in a range from 4 to 12, and the curvature radius ratio R1/R2 is set in a range from 2.6 to 4.6 when the aspect ratio H/SW is 0.55 or less. The tread surface is provided with a main circumferential groove, preferably having a groove width of 0.06 to 0.10 times the tire width SW, extending near the intersection of the first arc and the second arc in the tire circumferential direction for dividing the tread surface into the crown part and an outer shoulder part. Narrow lateral grooves crossing the tire circumferential direction divide the crown part and shoulder part into blocks.