The proportion of variance for service ram effects was estimated for number lambs born and weaned. The database with 11,311 lambings in purebred Suffolk was used. The basic model equation for the analysis of variance of litter size contained effects of ewe´s age at lambing, contemporary group of ewes at lambing, ewe´s permanent environmental effect and ewe´s direct additive genetic effect. The other models were extended by contemporary group of ewes during mating (harem), and additive genetic and permanent environmental effect of service ram. Variance components were estimated by the Gibbs sampling method. The proportions of variance for the service ram effect for number of lambs born and weaned were 4.1 % and 2.6 %. The annual genetic trends were 0.4 % of lambs born and 0.2 % of lambs weaned for female fertility. Male contribution on litter size was 0.2 % of lambs born and 0.1 % of lambs weaned. The results demonstrated that service rams in Suffolk sheep have low influence on litter size of their mates.