The effect of chitosan as a capping agent on the synthesis of ZnS nanoparticles via wet chemical route was studied. The used of chitosan as a capping agent for ZnS nanoparticles had allow the synthesis to be carried out in water which is a more simple and environmental friendly. In dilute acetic acid, the amines in the chitosan will protonated to become NH3+ ions. The positive charges around the ZnS nanoparticles are then used to disperse the nanoparticles via electrostatic repulsion. Chitosan also has an effect on the size of produced nanoparticles. From the X-ray analysis found that the crystallite size of nanoparticles decreases with the increasing of the chitosan concentration. The increase in chitosan concentration will promote the growth of the nanoparticles and subsequently produced smaller nanoparticles.