Organic nanoparticles of cholesterol, Rhovanil, and Rhodiarome have been synthesized in different microemulsions (AOT/heptane/water; Triton/decanol/water; CTABr/hexanol/water) by direct precipitation of the active compound in aqueous core under continuous ultrasound treatment. Generally, the size of the nanoparticles depends on different parameters such as the concentration of the organic molecules and the diameter of the water cores which is related to the ratio R ) [H2O]/[surfactant]. The nanoparticles are observed by transmission electron microscope after a treatment with iodine vapor. The particles remain stable for months. The influence of the concentration of the organic molecules in the range between 50 and 400 g/L is studied. The average diameter of the nanoparticles does not vary much and is between 50 and 70 Å. The average size of the particles does not change much as a function of the ratio R (except in the case of cholesterol nanoparticles) or the concentration of the organic molecules. This can be explained by the thermodynamic stabilization of a preferential size of the particles. The reaction of precipitation has also been carried out using a magnetic stirrer. The size of nanoparticles is somewhat greater than that obtained by using a ultrasound bath. Different solvents are used to solubilize the organic molecules, but the average diameter of the particles does not change. The solvents are considered as a carrier vector for the organic molecules and do not influence the precipitation in a significant way.