The actions of four organic acids in Radix isatidis, a traditional Chinese medicinal (TCM) herb, on Escherichia coli, Staphylococcus aureus and Shigella dysenteriae growth were investigated by microcalorimetry. The four organic acids were syringic acid, 2-amino-benzoic acid, salicylic acid and benzoic acid. The power-time curves of Escherichia coli, Staphylococcus aureus and Shigella dysenteriae growth with and without organic acids were acquired, meanwhile the extent and duration of inhibitory effects on the metabolism were evaluated by growth rate constants (k1, k2), maximum heat-output power (Pm) and peak time (tm). The inhibitory activity varied with different drugs. The sequences of anti-microbial activity of the four organic acids on Escherichia coli, Staphylococcus aureus and Shigella dysenteriae were all: syringic acid > 2-amino-benzoic acid > salicylic acid > benzoic acid. And benzoic acid promoted the growth of Staphylococcus aureus and Shigella dysenteriae. This study provides a basis for the further study on Radix Isatidis.