Abstract: In order to ascertain the causes of high mortality in calves in Tibetan agricultural areas,etiological and serological tests were carried out in the laboratory after field investigation and autopsy of diseased and dead cattle.Malnutrition resulted in low immunity of calves,which caused the invasion and rapid propagation of the pathogenic agents,and ultimately led to death.The etiological and serological tests showed that the fatality rate of calves in Tibetan agricultural areas was 9.34%.The highest and lowest mortality rate of calves were observed in Changdu and Lhasa,respectively,which were 10.10%and 8.50%,respectively.In the blood and tissue samples recovered from the weak,diseased and dying calves in Tibetan agricultural areas,the average positive rate of Escherichia coli and Salmonella was 48.05%and 31.54%,respectively.The serum antibody levels against some pathogenic agents of the dying calves were found higher than 98%by ELISA and agglutination test.This study provides theoretical references for control of the calf death in Tibetan agricultural areas.