Thirty-eight isolates of actinomycetes have been isolated from samples which were collected from mangrove ecosystem in Pramuka Island, Kepulauan Seribu, Jakarta, Indonesia. Isolation was done by using plate method on Zhang' Starch Soil Extract (ZSSE) agar. The pure cultures were grown in Cross Streak Medium (CSM) agar plate and subjected for antibacterial activity. Antibacterial activity was screened using plug method against Gram negative Escherichia coli NBRC 3301, Gram positive Staphylococcus aureus NBRC 100910 and Kocuria rhizophila NBRC 12708 then was determined by antibacterial Activity Index (AI). The result showed that three isolates inhibited Escherichia coli NBRC 3301 (0.408 - 0.708), 18 isolates inhibited Staphylococcus aureus NBRC 100910 (0.977 - 3.482), and 17 isolates inhibited Kocuria rhizophila NBRC 12708 (1.182 - 6.814). Among the strains tested, SRM 2 and SRM 13 were the most potential strains because both give positive result to all tested bacteria.