Sulfur-containing polymer supported palladium nanoparticles (Pd/SPMB) were prepared by using the cross linked poly(N,N'-methylene bis(acrylamide)) (SPMB) as the platform. The SPMB was synthesized via the reversible addition fragmentation chain transfer (RAFT) polymerization, from which the sulfur inherently exists in the RAFT polymer due to the use of thiocarbonate as the chain transfer agent, and could be further utilized as a modifier of Pd nanoparticles. The Pd/SPMB showed size-dependent activity and selectivity in the semihydrogenation of alkynes to alkenes. The catalyst with an average Pd size of 5.3 nm was active in the semihydrogenation of both internal and terminal alkynes. Most of the surface of Pd NPs was covered (or poisoned) by sulfide and thiolate due to the moderate complexation strength between Pd NPs and SPMB, thus leading to the selectivity enhancement toward alkene. Furthermore, the catalyst was highly stable and can be reused 10 times without an appreciable loss in either activity or selectivity.