The methanol, dichloromethane, hexane, chloroform, and volatile components of Dictyopteris membranaceae (Stackhouse) Batters (Dictyotaceae) and Cystoseira barbata (Good et Woodw.) J. Agardh (Cystoseiraceae) were tested for their antimicrobial activities (four Gram-positive bacteria, four Gram-negative bacteria, and Candida albicans ATCC 10239). Five compounds were identified in the volatile oil of D. membranaceae accounting for > 85% of the composition of the volatile oil. Twenty-eight compounds were identified in the volatile oil of C. barbata accounting for > 67% of the composition of the volatile oil. Major components were 6-butyl-1,4-cycloheptadiene (43.21%) for D. membranaceae and docosane (7.61%) and tetratriacontane (7.47%) for C. barbata . Many compounds in the volatile oil of C. barbata were identified as hydrocarbon compounds. The volatile oils of these algae did not remarkably inhibit the growth of tested microorganisms. However, the hexane extracts showed more potent antimicrobial activity than methanol, dichloromethane, and chloroform extracts.