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A crossbreeding experiment involving four Iranian fat-tailed sheep breeds, namely Kurdi (K), Chaal (C), Afshari (A) and Sanjabi (S) was carried out to determine the ram breed effects on productivity of Kurdi fat-tailed ewes (K) under extensive production system. A total of 475 Kurdi ewes were mated to 24 rams from four breeds producing 454 lambs including pure Kurdi, KK, (116 lambs), Afshari × Kurdi, AK, (113 lambs), Chaal × Kurdi, CK, (115 lambs) and Sanjabi × Kurdi crossbreds, SK, (110 lambs). Ram breed had a significant influence on early growth traits of the lambs (P < 0.01). The lambs sired by C rams were significantly heavier at birth and weaning than the other lambs (P < 0.05). Weight at 180 days of age (W180) in crossbred lambs was significantly higher than that of the purebred lambs (P < 0.05) while there was no significant difference among crossbred lambs for W180. Greasy fleece weight of lambs at first shearing (GFW) was significantly affected by ram breed (P < 0.05). Ram breed significantly influenced the fat-tail measurements of the lambs (P < 0.01). Number of lambs born and number of lambs weaned per ewe lambed or per ewe joined were not significantly influenced by ram breed effect, but the effect of ram breed on litter weight at birth and weaning per ewe lambed or per ewe joined was significant (P < 0.05). In general, ewes mated to Chaal ram had higher productivity than those mated to other ram genetic groups.