Genetic parameters for litter size traits were estimated for Czech Large White and Czech Landrace using an animal model. Herd-year-season, the permanent environmental effect of sow, mating type, age of sow at first farrowing (for first litter) or farrowing interval (for further litters) and the additive genetic effect of sow were the main factors in the model. The effect of including service sire as a further effect into the model was investigated by comparing three models; without service sire effect, with service sire effect taking or not taking into account the relationship matrix for service sires. Heritability estimates were similar in all three models (16 to 20 % for number born, 16 to 19 % for number born alive and 13 to 16 % for number weaned). The proportion of variance due to service sires was only 2 to 3%; therefore the effect of service sire can be omitted when estimating breeding values for litter size traits.