The data analyzed in the study were collected from the results of performance-testing on Corriedale, Polwarth, Romney Marsh and breed-crosses at the Namweon Branch of the National Animal Breeding Institute from January, 1980 to May, 1985. Theresultswere summarized as follows: 1. Mating systems affected significantly body weights at birth and weaning (P$lt;0.01). The least-squares means estimated for birth weight were. 3.72㎏ in pure bred Polwarth, 3.90Kg in Romney Marsh × Polwarth crossbred, 3.94㎏ in Pure bred Corriedale, 4.21㎏ in Romney Marsh × Corriedale crossed and 4.47㎏ in pure bred Romney Marsh; the corresponding for figures for weaning weights were 17.08㎏, 19.43㎏, 19.82㎏ and 20.97㎏, respectively. 2. Heritability estimates for weaning weight were 0.44±0.135 in corriedale and 0.41±0.204 in Polwarth.