We report a new strategy for the crosslinking of low density polyethylene (LDPE) by using a small amount of functional nanostructured hybrid agent, octavinyl polyhedral oligomeric silsesquioxane (OVPOSS), as the crosslinker, which dramatically decreases the amount of dicumyl peroxide (DCP) and thus avoids the chain scission, scorch, and production of small pores in the conventional peroxide crosslinking strategy. By melt blending under an extruder, OVPOSS aggregates with sizes of tens of nanometers are homogeneously distributed in the matrix of LDPE, as confirmed by SEM. FTIR, DSC, and rheometry are used to study the crosslinking process and product properties. We found that vinyl groups of OVPOSS are firstly activated by the initiator of DCP and then react with LDPE to form an integrated network. The crosslinking process is fast and highly efficient because each OVPOSS molecule has eight reactive vinyl groups. The reactions complete within 10 min at 175 °C, and speed up with the increase in the content of DCP or OVPOSS. In the presence of 0.2 phr DCP, 0.5 wt% OVPOSS can effectively crosslink the composite sample with comparable properties to that with 2 phr DCP yet without OVPOSS. The obtained crosslinked LDPE should be suitable for high voltage cable materials. We believe that the approach using functional agents is powerful to crosslink or functionalize other polymers for special properties and applications.