The study analyzed in the study were collected from the results of performance-testing on Corriedale, Polwarth at the Nawweon Branch of the National Animal Breeding Institute from January 1981 to May 1985. Birth weights, weaning weights, body weight at the age of 18-month were analyzed; and constants and least-squars means for the traits were estimated by the least-squares methods. The results are summarized as follows: 1. Years of birth affected significantly body weights at birth, at weaning and at the age of 18-month in corriedale and Polwarth(p $lt;0.01). The constant estimates of years of birth for birth weights ranged -0.167∼0.208㎏ in corriedale and -0.113∼0.089㎏ in Polwarth, and the corresponding figures for weaning weights were estimated -1.655∼1.509㎏ and -1.983∼2.051㎏; and the estimates for body weight at the age of 18-month were -1.982∼2.746㎏ and -1.757∼1.884㎏, respectively. 2. The effects of ages of dam were significant for birth weights and weaning(p $lt;0.01), but were not influential for body weights at the ages of 18-month. The constant estimates of ages of dam ranged -0.124∼0.079㎏ for birth weights and -0.664∼0.631㎏ for weaning weights in Corriedale, and the corresponding estimates in Polwarth were -0.133∼0.086㎏ and -0.628∼0.921㎏ respectively. Lambs from above two-year-old matured ewes were heavier than those from two-year-old ewes. 3. Sex of the individual lamb greatly affected birth weights and weaning weights in Corriedale and Polwarth, but no significant differences were found between lamb sexes in birth weights in polwarth compared with the birth and weaning weights of ewe-lambs those of ram-lambs were 0.08㎏ heavier in Corriedale and 0. 06㎏ heavier in polwarth, and weaning weights of ram-lambs were 1.22㎏ heavier in corriedale and 1.02㎏ in Polwarth. 4. Types of birth had significant effects on body weighs at birth and weaning in Corriedale and Polwarth(p $lt;0.01). Singles were heavier than twins by 0.68㎏ in corriedale and 0.48㎏ in polwarth at weaning In body weights at the age of 18-months singles were superior to twins by 0.54㎏ in Corriedale and by 2.84㎏ in Polwarth, and a significant difference was detected only in polwarth