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In this study, the data of birth weight (BW), weaning weight (WW) and daily gain from birth to weaning (DG) of Lori-Bakhtiari lambs were used to estimate effects of environmental factors and heritability, phenotypic and genetic correlations amony these traits. The effect of age of dam on birth weight trait was highly significant (P <0.01). The BW lambs born to 2-year-old ewes compared with lambs born to other age groups (3 to 6 and 7-year-old) were significantly lighter. The male lambs compare to the female ones had a significantly heavier at birth, weaning and also they had a higher daily gain from birth to weaning.A comparison between lambs born single and twin indicated that the BW, WW and DG of lambs born single were significantly higher. Lambing year had a highly significant effect on the preweaning traits.
Heritability of BW, WW and DG were 0.114±0.080, 0.105±0.084 and
0.106±0.084, respectively. Phenotypic and genetic correlations between BW and WW, BW and DG and WW and DG were 0.236 and 0.493(±0.414), 0.123 and 0.436(±0.498), 0.994 and 1.000(±0.000), respectively.