Blood samples of 960 buffaloes in 4 provinces (Lang Son, Bac Kan, Hoa Binh and Son La) were examined by several methods (wet blood film, Giemsa stain, Slide agglutination test, Card agglutination tests) for determining the prevalence of Trypanosoma. The obtained Trypanosoma strains were analyzed, categorised and determined for biological characteristics and virulence in a number of experimental animals including buffalo calf, rabbit, white mouse and dog. The studied results showed that: The Trypanosoma strains isolated from buffaloes all having the typical morphological characteristics of Trypanosoma evansi. The prevalance of T. evansi in buffaloes in the four above mentioned provinces was 12.60 percent. The highest infection ratio was in Bac Kan (14.10 percent) and the lowest was in Lang Son (11.66 percent). The highest infection ratio according to the age groups was at the age of 4 - 8 years old (12.50 percent). The main clinical symptoms in buffaloes infected with Trypanosoma evansi were: prolonged diarrhea (44.11 percent), lean body condition, weakness, depression relatively high proportion (52.94 percent). paralyzed legs walking difficulty (8.82 percent.), miscarriage (2.94 percent.), intermittent fever (17.64 percent). conjunctivitis and cornea (35.29 percent). The carriers (healthy animals) have no symptoms accounted for relatively high proportion (58.82 percent). The diagnostic method based on challenge test in white mouse could be able to detect T. evansi in the blood soon (2 days after infection). The blood physiological indicators such as number of red blood cells, white blood cells in buffalo calfs were reduced after 8 week challenge test. Virulence test of Trypanosoma evansi in animals indicated that mice were the most susceptive compared to the other experimental animals. Both trypamidium and azidin were effective therapeutics for the infected cases.