This study focused on the comparison of the growth characteristics of single and twin birth lambs in Karayaka sheep which is an indigenous breed of the northern part of Turkey. Gompertz growth function was fitted to body weight age data of 81 lambs (39 males and 42 females) from birth to 10 months of age. Single birth lamb of both sexes showed lower asymptotic weight than the twin birth ones. There were a noticeable difference in the absolute growth rate between birth types before inflection point but decline after the inflection point was slower for twins than that for singles. Similarly, the decrease in relative growth rate was higher for singles than that for twins. The Gompertz model parameters showed similar trends for birth types in both sexes. The results indicated that the type of the birth should be taken into account besides the sex of the individuals, while working on biological modelling of sheep growth and subsequent genetic evaluations of the related traits. © Medwell Journals, 2010.