The objectives of these studies were to investigate effects occurring during a mating season, select the proper model for genetic parameter estimation and to estimate genetic parameters for litter size in sheep. Variance components and genetic parameters for litter size were estimated using the BLUP animal model with repeatability. The basic model equation contained ewe age as a fixed effect and random effects of contemporary group, permanent environment and direct additive genetic effect of the animal. Modifications of the basic model were examined when various combinations of mating effects were included (contemporary group of ewes during mating (harem), additive genetic and permanent environmental effect of service ram). The effect of service ram ranged from 0.01 to 0.02 of phenotypic variance in Šumava sheep, in Romanov sheep it was 0.05 – 0.10, and in Suffolk sheep it was 0.04 – 0.05. Including effects of mating (service sire, harem, and/or ram´s permanent environmental effect) in the model decreased deviance information criterion, what means that these models are more proper than the basic one. Results from these studies demonstrate that the service rams have low but a clearly detectable influence on litter size of their mates. Genetic parameter estimates indicate that direct selection on the service ram effect could increase litter size and achieve genetic gain through ram selection.