Silver - chitosan (Ag@CS) nanoparticles were synthesized by in-situ encapsulation method, which achieved an average size of 12.6 nm, uniform particles, well-dispersed particles in water and stable. Experimental results show that the temperature and reaction time have greatly affected on the formation of Ag@CS nanoparticles, particle size and uniformity. The optimal temperature is 60oC during 3 hours to obtain Ag@CS nanoparticles with Ag content of 9.01%. The zeta potential of Ag@CS in the colloidal form obtained after 3 months has a value of +25.6 mV, proving that Ag@CS particles have good stability, opening up the potential for using this material system as a plant protection product.