The gelation process of N-Phenylaminomethyl-POSS/PU (polyurethane) nanocomposites during curing and at the stable state after curing was investigated by rheology. An increase in dynamic shear moduli, G' and G '', was observed during the dynamic temperature ramps of the sample. In time-resolved mechanical spectroscopy (TRMS), G' and G '' increased with curing time at constant curing temperatures over a wide of frequencies. The gelation time of the composites decreased with the rise of curing temperature or with the increase of POSS concentration. The relaxation exponent n at the critical gel was around 0.73. The gel stiffness S decreased as curing temperature increased or as PUSS concentration increased. After the completion of the curing reaction, the critical concentration of PUSS beyond which the gelation of POSS/ PU composites would happen was found around 2.5 wt%. The viscoelastic properties of crosslinking POSS/PU fitted time temperature-superposition well which implied the incorporation of multifunctional PUSS into PU increased the homogeneity of crosslinking POSS/PU composites. Surprisingly, the modulus of the fully cured materials between 2.7 wt% and 6 wt% could also be supposed onto a master curve at high temperature, which implied self-similarity of network near the critical gel. The similar microstructure of POSS/PU at stable state was also confirmed by TEM. The network formation mechanism and the fine structure of the crosslinking POSS/PU were firstly investigated which would provide technical and theoretical basis for other hybrid crosslinking systems. (C) 2014 Elsevier Ltd. All rights reserved.