The reaction behavior of atom transfer radical polymerization (ATRP) with crosslinking was studied using di(meth)acrylates as the model network-forming system. The kinetics was followed by differential scanning calorimetry (DSC). The structural evolution of the networks was investigated by gel permeation chromatography (GPC) and extraction/swelling measurements. The linear primary chains were obtained by hydrolysis of the networks and analyzed by viscometry. It was found that the polymerization proceeded in a controlled/living manner in an early stage of the reaction with the molecular weight of primary chains increasing linearly with vinyl conversion. However, deviations from the linearity occurred at high monomer conversions due to the network structure that severely restricted mobilities of catalyst/ligand complexes in the radical activation/deactivation. The onset and degree of deviation depended on crosslinking density. The diffusion-controlled radical deactivation caused a mild autoacceleration in the polymerization rate. Adding excess Cu(II) to the system reduced the polymerization rate but improved the linear growth of primary chains that would facilitate the synthesis of networks of controlled microstructure.