Chemical compositions in the leaf essential oils of four Syzygium species were first identified by the GC-FID/MS analysis. Monoterpene hydrocarbons (39.6-51.1%), and sesquiterpene hydrocarbons (27.0-37.8%) were the main chemical classes in S. formosum and S. syzygioides leaf oils, whereas sesquiterpene hydrocarbons (83.0-83.8%) were predominant in S. Megacarpum and S. chantaranothaianum leaf essential oils. Bicyclogermacrene (9.1-37.0%) was the principal compounds in these essential oils. All the tested essential oils with the MIC values of 16-128 µg/mL were comparable to the positive control streptomycin against Gram-positive bacteria Staphylococcus aureus and Bacillus cereus, and Gram-negative bacterium Pseudomonas aeruginosa. The leaf essential oils of S. formosum, S. syzygioides, and S. chantaranothaianum with the MIC values of 16-128 µg/mL were comparable to the positive control cycloheximide against the yeast Candida albicans. Four samples also exhibited good larvicidal activity against the mosquito Aedes aegypti with the 24-h and 28-h LC50 values of 25-40 µg/mL