Black Slavonian breed of pig is usually kept on family farms, that has different keeping conditions. Environmental effects on fertility are more obvious than in intensive systems. Aim of the study was to determine and analyse the factors that affect the fertility of the Black Slavonian Pig. There were 10 233 records of fertility of Black Slavonian pig analyzed, collected on 3 191 animals in period from 1997 to 2015. Variables in data set included breeding organization, identification number, serial number of sows farrowing, farrowing date, identification number of boars - father of litter, date of mating, number of piglets born alive. The final data set for analysis of factors litter size contained 10 127 records of farrowing. The impact of the breeding season due to climatic variations within the year was not significantly expressed, which can be attributed to the fact that the litter size of Black Slavonian pig is low. The long termed effect of mating season was more obvious. The research confirmed that the number of piglets born alive rises to fourth litter with additional increase after seventh litter. Variability between litters of sows admits caused by different boars is observed, and the difference between highest and lowest estimate is 10 piglets.