Population parameters such as effective population size and coefficients of inbreeding are important information of a population but have rarely been studied in Santa Inês sheep. Therefore, this study aimed to estimate population parameters in a Santa Inês sheep flock and the inbreeding effect on performance traits. A dataset with 11,564 animals, born from 2003 to 2011, was recorded for weights at birth (BW1), 60 (BW60), 180 (BW180) and 270 (BW270) days of age, daily weight gain from birth to 60 d (DWG1), 60 to 180 d (DWG2), and 60 to 270 d (DWG3). Percentages of animals with known pedigrees decreased over generations, from 70 % in the first generation to less than 5 % in the third. The effective population size decreased from 665 in 2004 to 45 in 2010. The effective number founders and ancestors were 285 and 273, respectively. Furthermore, the average relatedness coefficient was 0.47 %. The highest frequency of inbred animals was concentrated between 0 and 10 % of the inbreeding coefficient and only 263 animals