Data on 3984 lambing records of 1285 Lohi ewes kept at the Livestock Experimental Station, Bahadurnagar, Okara for the period 1980-2011 were analyzed by using Harvey’s Mixed Model Least Squares and SPSS Computer Programme. The purpose was to estimate the magnitude of various environmental sources of variation influencing birth weight in this breed of sheep. The least squares mean for birth weight was 3.60± 0.03 kg. The trait was significantly (P< 0.01) & (P< 0.05) influenced by the year and season of birth, type of birth and the sex of the lamb born. The age of the dam also had significant effect on birth weight of the lamb; the younger ewes produced lighter (3.48 ± 0.03 kg) lambs than older ewes (3.69 ± 0.03 kg).