Abstract: In order to assess the cause of death of lambs in northwest Tibet pastoral area in cold season,investigations were conducted for three consecutive years in this region.The tissue or blood samples and serum samples were collected from the diseased or dead lambs and were used to perform etiological agent isolation and antibody determination,respectively.The results showed that the mortality of northwest Tibet lambs in three representative counties Rutog,Nyima and Gaize was approximately30.00%,the fatality rate of sheep lambs and goat lambs were 29.90% and 30.57% respectively,and the fatality rate of one-to twomonth-old lambs were above 30.00%;for the weak,diseased and moribund lambs aged from two day to sixty days,the average positive rate of E.coli in blood test was 25.42% and that of Salmonella was 17.82%;for the dead and moribund lambs,the average positive rate of E.coli,Salmonella and coccidia egg in blood test was 52.66%,28.02% and 11.59%,respectively.The results of this study suggest that the etiological factors associated with the death of northwest Tibet lambs were complex,and the comprehensive prevention and treatment measures are expected to be taken to effectively control the death of lambs in this region.