The aim of this study was to investigate the antimicrobial activity of Gac fruit (Momordica cochinchinensis) extract against gram positive and gram negative bacteria using agar disc diffusion method and lytic activity assay. The parts of seed pulp and flesh of Gac fruit were extracted using 95% ethanol, ether and distilled water as a control. The result showed that the extract of flesh with distilled water inhibited gram positive bacteria more than gram negative bacteria as well as the extract of flesh with ethanol. The extract of flesh with distilled water showed significant inhibition against Staphylococcus aureus, Planococcus sp. and Micrococcus luteus with inhibition zone of 10.0, 9.2 and 9.0 mm, respectively. The maximum antibacterial activities were observed on Micrococcus luteus 745 in ethanolic extract of flesh (20.0 mm). However, the extract of flesh with ether had no antimicrobial activity. Moreover, the extract of seed pulp with ethanol showed strong inhibition against Micrococcus luteus (19.0 mm) which are gram positive bacterial strains. The extract of seed pulp with ether had no antimicrobial activity. For the Lytic activity against Staphylococcus aureus, the extract of seed pulp with ethanol showed the highest value of 22.82%, followed by the extract of seed pulp with distilled water at 22.51%, while the extract of flesh with ethanol was 11.57% and the extract of flesh with distilled water was 10.78%, respectively.