Acyclovir is also known as Acyclovir, this drug is used for treatment of viral infection, particularly for treatment of herpes simplex viral infection. These are taken for month during treatment of herpes simplex viral infection. These are showing the action against all herps virus family. The acyclovir is poorly water-soluble drug. Due to that main aim is to increase the solubility of acyclovir in other solvent. The bioavailability of acyclovir is very less about (15-35%) because it has less oral route absorption. Due to that the acyclovir are given in intravenous route. When acyclovir is taken in oral route, the peak plasma concentration occurs after 1-2 h. The acyclovir having 9-33% of plasma protein binding. The BCS class of acyclovir are Class third (high solubility and Low permeability). Due to that acyclovir are formulate in the form of nanoparticle. Chitosan are the polymers which are used for the formulation of nanoparticle. The chitosan is found to be compatible with acyclovir. Formulation of acyclovir nanoparticle was done by Nano-precipitation method. Many evaluation tests performed during the formulation of Acyclovir nanoparticle mainly zeta sizer is use for the determination of particle size, zeta potential and PDI (poly disperse index) also performed evaluation of loading efficiency and % Drug entrapment. Keywords:  Acyclovir, Chitosan, Zeta sizer and Nanoparticle.