Central to the concept of seed-mediated growth of nanoparticles is that small nanoparticle seeds serve as nucleation centers to precisely grow nanoparticles to a desired size. Herein we present an environmentally benign and size-controlled seed-mediated growth method coupled with an organic phase extraction to yield monodisperse dodecanethiol capped Pd nanoparticles. Monodisperse Pd nanoparticles (ca. 3.4 nm, standard deviation: 0.6 nm) were synthesized and stabilized by a “green” and inexpensive polysugar, sodium carboxymethyl cellulose (CMC) in aqueous solution. The monodisperse Pd nanoparticles were employed as seeds on the surface of which fresh Pd ²⁺ ions were reduced by weak reducing agent ascorbic acid. This seed-mediated growth procedure results in a complete separation of nucleation and growth, and hence, the atomic-level size controlled synthesis of the Pd nanoparticles. Using different salt concentration in this seed-mediated growth process, different sized Pd nanoparticles were controllably synthesized. The nanoparticles could then be extracted from the aqueous solution to an organic phase by using dodecanethiol as extractant with the assistance of concentrated HCl. The advantage of the seed-mediated growth method used in this study is that the Pd nanoparticles size could be precisely controlled (from 3.4 nm to 9 nm) by simply varying the ratio of seed to Pd salt. Furthermore, the use of solvent (water), capping agent (CMC) and reducing agent (ascorbic acid) in the system provides the “green” and cost-effective merits for this precise size-controlled particle synthesis method.