Marine algae are known as source of bioactive secondary metabolites. Green marine algae (Ulva intestinalis) and red marine algae (Gracilaria fisheri) were collected from the coast of Pattani province, Thailand. These marine algae were extracted by four solvents including methanol, ethanol, dichloromethane and hexane. Crude extracts of all seaweed samples were tested for their antimicrobial activities using disk diffusion method and colorimetric broth microdilution method, respectively. Thirteen bacterial strains were used in this study, Vibrio alginolyticus PSU VA 1, V. parahaemolyticus PSU 5124, V. harveyi PSU 4109, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Klebsiella pneumoniae, Salmonella typhi, Proteus mirabilis, Staphylococcus aureus ATCC 29213, Listeria monocytogenes DMST 4553, Bacillus cereus TISTR 687, Methicillin resistant S.aureus NPRC 001R (MRSA 001R) and Enterobacter faecalis ATCC 29212. All crude extracts showed ability against tested strains (ranging of inhibition zone 6.78–16.45 mm), with the exception of the U. intestinalis dichloromethane extract in the disk diffusion assay. Dichloromethane and hexane extracts of G.fisheri showed the highest antimicrobial activity against S.aureus ATCC 29213 and B. cereus TISTR 687 with minimum inhibitory concentration (MIC) values of 256 μg/ml, respectively. From this finding confirm that the extracts of algae are good source of bioactive metabolites.