Palladium nanoparticles (PdNPs) are one of the most attractive metal nanomaterials because of their excellent physicochemical properties. PdNPs have been studied for many different applications such as Suzuki cross-coupling reactions, hydrogen purification/storage/sensing, CO oxidation, fuel cells, prodrug activation, and antimicrobial therapy. Recently, PdNPs have been explored as photo-absorbers for photothermal therapy and photoacoustic imaging in the treatment of cancer disease. Herein, we reported a scalable, efficient, green, and one-step method to synthesis PdNPs. The chitosan polymer was used as a stabilizer and vitamin C was used as a reducing agent. Interestingly, the reaction temperature can be adjusted to the size of PdNPs. When the reaction temperature was increased from 25^oC to 95^oC, the morphology of resulted PdNPs changed from flower shape to spherical shape and their nanoparticles sizes decreased from 64 nm to 29 nm. The characterization revealed that the obtained PdNPs were relatively uniform in size, shape and stable in aqueous solution.