The aims of this study were to estimate the inbreeding level and to evaluate inbreeding effects for birth (BW) and fourth week weight (FWW) in a multi-breed sheep population. The analyses were performed within eight lines and the total population. The data set of 36488 pedigreed individuals over twenty years was analysed. The inbreeding coefficients were derived from an additive relationship matrix. The inbreeding depression was expressed as a partial linear regression coefficient estimated via a single trait animal model including fixed effects (flock, type of birth, sex, breed, inbreeding coefficient - as linear covariable) and random effects (additive genetic and residual). The number of pedigreed individuals varied considerably over time. The study has shown that the inbreeding of this population was very low (0.30%). However, the estimate is considerably influenced by incomplete pedigree information. Comparative analysis of inbreeding levels within lines and breeds indicated on differences between them. Both positive and negative inbreeding effects (ranged from -12.6 to 16.0 g for BW and from -61.9 to 18.1 g FWW) were found depending on line.