Free anino acid contents in alcohol extract of eight plants of Caryophyllaceae and their microbial activities were investigated. 1) Amino acid contents in both of the Pseudostellaria palibiniana and Stellaria media was the highest among them and the contents was less in the order of Cerastium caspitosum and Stellaria aquatica. 2) Of all free amino acids contained in eight plants, valine was the richest, and then glutamic acid, leucine in that order. On the other hand, no methionine was observed and cystine, lysine and histidine were found in small amounts. 3) Of eight plants exhibited good antibacterial action against Sarcina lutea, B. subtilis and Sal. typhi. 4) S.aquatica and Pseudostellaria palibiniana showed antibiotic actions against all bacteria except for fungus, Candida albicans. 5) C. caspitosum and C. brachypetalum showed inhibition zone against B. subtilis and Sal. typhi only. 6) Antibacterial activity against gram(+) bacteria was more potent than gram(-).