We prepared the organic nanocrystals by the reprecipitation method using a carbosiloxane dendrimer. The dispersing process can be controlled by the carbosiloxane dendrimer which presented good site-isolation effect in host-guest chemistry. The size and shape of the controlled nanocrystals were analyzed by UV/vis spectroscopy and SEM. The absorbance of UV/vis spectra varied during the evolving process. The shape was more regular and the size can be well controlled under 100 nm. The results indicated the optical properties of organic nanocrystals strongly depends on their sizes.