Tetra-poly(ethylene glycol) (PEG)-based nanocompositc hydrogels (tP-NC gels) were prepared by in situ polymerization of two kinds of macromonomer (TN-PEG and TA-PEG), each having four reactive PEG arms, in the presence of clay (Laponite XLG) in aqueous media. By adopting appropriate synthetic conditions such as the use of pyrophosphate-Na buffer and selected homogeneous mixing procedures, tP-NC gels with high transparency and excellent tensile properties, e.g., high elongation (900-1000%) and high tensile strength (300-560 kPa), approximately 2-4 times those of the corresponding original tetra-PEG gel, were obtained at a relatively low clay concentration (C-clay=2 x 10(-2) mol/L-H2O) and a wide range of polymer concentrations (C-p=120-240 mg/mL-H2O). We investigated the effects of buffer, C-clay, and C-p on forming tetra-PEG/clay gels, their tensile properties, and the network structures. Also, we studied interactions between the clay and tetra-PEG segments, by measurements such as viscometry, X-ray fluorescence, transmission electron microscopy, and Fourier transfer infrared spectroscopy. It was revealed that the pyrophosphate-Na buffer plays an important role in dispersing exfoliated clay uniformly in the reaction solution and that the clay platelets incorporated into the tetra-PEG networks interact with a specific ester group in the tetra-PEG arm as well as with amide linkages in the tetra-PEG networks so as to improve their tensile properties.