A heavy-duty power transmission V-belt is so designed that respective upward projections each having an arcuate cross section of respective blocks are fixedly engaged with respective bottom-surface grooves each having an arcuate cross section of a tension member. The radius of curvature R2 of each bottom-surface groove is set larger than the radius of curvature R1 of the upward projection of the block (R2>R1) in the state that the tension member is put strait. Thereby, when the V-belt bends around a pulley, it can be avoided that a large local strain occurs at the bottom-surface groove. This allows a firmly fixed state of each block by the tension member to be maintained for a long time.