Chitosan has been used extensively in many different fields due to it's biocompatible, non-toxic nature, anti-microbial, anti-fungal and antioxidant activities. The nanochitosan particles inherit not only the properties of chitosan but also possess other special characteristics since small size and large surface area. Some reports have been published on the antimicrobial activity of nanochitosans but very few of correlatil)n between particle size and their antibacterial activity. The authors have created 3 types of nanochitosan with different size by ionic gelation method changing stirring speed and stirring duration of chitosan : tripolyphosphate (TPP) mixture. When using the stirring speed of 4,000 rpm for two hours, the formed nanochitosan particles were about 1,000 nm, the increase of duration up to 4 hours, the size of particles decreased to a half, about 500 nanometers. Increasing the stirring speed up to 8,000 rpm for 1 hour, nanochitosan particles with size about 100 nanometers have been created. The antibacterial and antifungal activities of the formed chitosan nanoparticles has been evaluated. The results showed that received nanochitosans inhibited the growth of investigated bacteria and fungi stronger than chitosan solution. The smaller nanoparticles exhibited antimicrobial activity better than bigger one. The Gram (-) bacteria L. damsella was more sensitive to all types of nanochitosan particles than the Gram (+) bacteria (Bacillus and Micrococcus). Among three investigated fungi, A. awamori strains resisted chitosan solution as well as nanochitosan particles.