OBJECTIVE: To compare the dissolution property of Cassia angustifolia ultramicro decoction pieces and different powders in vitro. METHODS: Paddle method was used to determine the dissolution of effective constituents. HPLC method was developed for the dissolution amount of sennoside A and sennoside B in the different periods. We calculated the accumulative dissolu- tion rate, and fitted the fimction of Weibull distribution. RESULTS: There was a obvious difference in dissolution among C. angus- afolia plain powder, fine powder, ultramicro pow...