The aim of this research is to study the accuratly age of Bali cattle vaccination so that could obtained the optimal cellular immune response against Jembrana virus disease. In this research was used three Bali cattles i.e. 7, 9 and 12 month of age respectively. Each of the cattles were adapted in two weeks and were given the anthelmintic treatment. The feeding was given three times daily and the drinking water ad libitum. The vaccine was inoculated by 3 ml JD.vacc.sp.15 vaccine (BBVet Denpasar production) by intramuscular route respectively. One month after first vaccination, was conducted booster by similarly dose and route respectively. One week after the booster, from the each cattle was taken its peripheral blood cells for to isolated their lymphocytes by picoll-paque gradient method. Each of this lymphocyte cells were cultured in non serum DMEM media for the MTT assay. MTT assay was used for to determine the cellular immune response. The mitogen was used whole protein of Jembrana disease virus. Result of the research showed significantly difference (p<0,05) of cellular immune response among the age 7 month with 9 and 12 month of cattles. There were not significantly difference (p>0,05) of the response between 9 with 12 month of age. This result indicated that the cattle which 9 month of age is the good timing to start the vaccination, because its cellular immune response is optimal.