The stress relaxation by addition-fragmentation chain transfer (AFCT) in highly cross-linked thiol-yne networks is investigated. The allyl sulfide AFCT functionality was incorporated in a thiol-yne system through the formulation of an allyl sulfide-based diethynyl monomer (2-methylenepropane-1,3-di(thiobut-1- yne), MDTBY, with a tetrathiol (pentaerythritol tetrakis-(3-mercaptopropionate), PETMP at a 1:2 ethynyl/thiol stoichiometric ratio. Thiol-yne network was fabricated utilizing a propyl sulfide-based diethynylmonomer(2-methylpropane-1, 3-di(thiobut-1-yne), MeDTBY. The stress that accumulates during a polymerization is typically attributed to the extent of postgelation shrinkage and the modulus of the polymerized material. The IR spectroscopy of PETMP-MeDTBY indicated rapid consumption of the ethynyl functional group. The photopolymerization of PETMP-MDTBY was expected to exhibit a much higher polymerization stress level as compared to PETMP-MeDTBY in the absence of any network relaxation associated with the AFCT-mediated relaxation mechanism.