The relationships between seed set and fruit weight and shape were studied on four nashi (Pyrus serotinaRehder varcultaRehder) cultivars Hosui, Kosui, Nijisseiki, and Shinsui. The carpel number ranged between four and eight per fruit with each carpel having the capacity to carry two seeds. The number of seeds per fruit was significantly correlated with fruit weight with ‘Nijisseiki’ showing the highest fruit-weight increase per seed. The seed number per carpel and the total weight of seeds per carpel were related to fruit shape. Even seed distribution around the fruit core was related to evenly shaped fruit. On the basis of the levels of seed set observed, considerable potential exists for raising the nashi quality by optimising pollination and seed set.