In herbal treatment of kidney stones, antilithics are used to "dissolve" the stones or aid their passing to guard against further retention. Diuretic action is also needed to increase the amount of fluid going through the kidneys and flush out the deposits. Previous clinical studies have shown that herbal medicines and their concoctions could be used to inhibit calcium oxalate crystallization. However, the pharmacodynamics and in-vitro effects of such medicines have not been established. Five Chinese herbal medicines were selected based on their usefulness in treating stone disease. A 96-well plate oxalate-induced turbidity in artificial urine was used to evaluate the efficacies of the different herbal medicines on calcium oxalate crystallization. The metastable limit was determined and the nucleation rate was derived from 12-min time-course measurement of turbidity at 405 nm. Phase-contrast microscopy was used to visualize the crystals. The results showed that with increasing concentrations of herbal extracts, smaller calcium oxalate crystal sizes were observed. Overall, the five herbal medicinal extracts tested were able to promote nucleation of calcium oxalate crystals while at the same time decreasing the size. This in-vitro crystallization confirms that prophylaxis of renal stones could be achieved by reducing overall supersaturation through promotion of small crystal nucleates and concomitant pharmacological diuretic action of herbal medicines. Clinical studies will provide more definitive conclusions.