Objective: To establish a method for the determination of the particle size of nifedipine and study the effect of particle size on the in vitro dissolution behaviors of nifedipine sustained release tablets. Methods: Light scattering was used to study the parti-cle size of nifedipine API. Nifedipine APIs with different particle sizes were prepared by a portable high-speed grinder. The in vitro dis-solution curve of nifedipine sustained released tablets (Ⅰ) was determined by HPLC. The similarity was evaluated using the similarity factor ( f2) with the original drug (trade name: Adalat-L, specification: 10mg) as the reference preparation. Results: The granulo-metric conditions were as follows: the pump speed of laser size analyzer was 1 800 r·min-1, the shading rate was 8%-20% , the bal-ance time was 0 s, the media was 0. 3% Tween 80, and the ultrasonic time was 1 min. The in vitro dissolution of nifedipine sustained released tablets (Ⅰ) showed that the smaller particle size of nifedipine API, the better the dissolution was. As the Dv90 ( the particle size accounting for 90% of the total particle quantity) was reduced from 118. 781 μm to 3. 471 μm, the cumulative dissolution in 0. 25 h of nifedipine sustained released tablets (Ⅰ) increased from 11. 2% to 44. 0% , the similarity factor ( f2) compared with the dis-solution cruve of the original drug increased firstly and then decreased, and f2value was 77 when the Dv90 was 29. 823 μm. Conclu-sion: The in vitro dissolution of nifedipine sustained released tablets is improved remarkably by micronization technology. In order to produce nifedipine sustained released tablets (Ⅰ) with the same bioavailability as the original drug preparation, the particle size of nife-dipine API should be controlled within the range of 15 μm≤Dv90≤45 μm.