In this study, the synergistic effect of the bark from Sumrong (Sterculiafoetida L.) extract with tetracycline was indicated against opportunistic gram negative bacteria : non drug resistant and drug resistant Pseudomonas aeruginosa and Escherichia coli ATCC 25922. The methanol extract of Sumrong mixed with tetracycline was designed for synergistic test. The antibacterial activity were tested by Agar diffusion susceptibility test, and also evaluated by FICI (Fractional Inhibitory Concentration Index). The results were revealed that the Sumrong extract with tetracycline indicated indifferently synergistic effect against drug resistant P. aeruginosa with FICI of 2. Also, these experiments indicated partially additive effect against E. coli ATCC 25922 with FICI of 0.5003.