A number of 5558 lambing records of 2245 Mehraban ewes, collected during years 1995 to 2010 were utilized to estimate the variance components and heritability for the number of lambs per lambing,  making use of B-Spline random regression models. The fixed factors affecting the studied trait were identified using general linear model analysis as well as SAS software, where flock-year-season and the age of ewe at lambing (month) had significant effects. Variance components were estimated as based on linear, quadratic and linear-quadratic B-Spline random regression models with 4 to 6 knots, using WOMBATsoftware. The models were compared using AIC and BIC indices, out of which model QLL4 (quadratic for fixed regression and linear for additive genetic, as well as permanent environmental effects with 4 knots) was found out as more suitable than the others. According to this model, estimates of additive genetic, permanent environmental, and phenotypic variances of a number of lambs per lambing stood in the ranges of 0.01 to 1.24, 0 to 4.09 and 0.34 to 5.98, respectively. Moreover, heritability estimate for the studied trait was in the ranges of 0.02 to 0.28. The heritability estimates for the number of lambs per lambing at 60 to 72 months of age were higher than those for the other ages.