The data used in this research were according to survival rate records of 9700 lambs from 735 rams and 8082 ewes, which collected at Moghani Flock Breeding Research Station during the year 1365 to 1390. The frequency distribution causes of culling and survival distribution function of lambs to yearling age were estimated by SAS (2000). Results revealed that 49.81 percent of total lambs were culled from the flock until the first year of life because of reasons including illness, lack of breed purity, etc. The mortality rate over this period was estimated 20.81 percent which occurred 8.03, 7.5, 4.43 and 0.85 percent in first three months, second three months, third three months and fourth three months of lambs life, respectively. According to percentage of mortality rate, the accumulative survival of lambs from birth to yearling age was calculated 79.19 percent. The regression coefficient of lambs survival from birth up to yearling age showed a monthly 1.77 percent decrease in lambs survival of this breed. The most mortality rate occurred in first, fifth, second, fourth, sixth and ninth months of lambs life, respectively. In comparison with third and fourth three months of lambs life, the fatality in first and second three months were higher because of heavy rainfall and frigid weather in winter quarters.