A kind of well-defined organic nanoparticle composite polyacrylamide (PAAm) hydrogels (OC gels) are successfully fabricated using in-situ free-radical polymerization with polystyrene (PS) nanoparticles as the cross-linker. These gels show excellent mechanical properties for the uniform dispersion of the organic cross-linker which is prepared by emulsifier-free emulsion polymerization. In addition, a particular swelling behavior is also revealed due to the introduced hydrophobic PS nanoparticles. Both the mechanical and swelling properties show obvious changes with various contents of PS nanoparticle and acrylamide (AAm) monomer in the initial reaction solution. The mechanism for OC gels formation is proposed, and it is validated that the hydrogen bonding interaction among polymer chains also plays an important role in the network formation in addition to the combination of the mutual termination of the attached radical chains on vicinal particles. Besides, the facile route is feasible to design stabilized and functional hydrogels with excellent mechanical properties without affecting the original properties of hydrogels.