Amorphous polyamide6/fumed silica nanocomposites are a new class of polyamide with improved barrier properties. Recently, polymer/fumed silica nanocomposites have received much attention due to the benefits of strongly improved physical, mechanical and chemical properties compared to pure polymer. In-situ polymerization of -Caprolactam in the presence of polyester polyol leads to the formation of amorphous polyamide6/fumed silica nanocomposites. Polyamide6 was prepared via polymerization of -Caprolactam as a monomer, sodium Caprolactam as the catalyst, polyester polyol as a comonomer, toluene diisocyanate as an activator and fumed silica as the nano particle. The objective of this research was to study the effect of various concentrations of polyester polyol and fumed silica on crystallization of nylon6 and formation of nanocomposites. Evaluations were made for the effects of various concentrations of catalyst, activator, comonomer and nano particle on initiation time of the reaction. Physical and mechanical properties of different formulations were prepared via reactive melt blending and evaluations were determined using the following techniques; Dynamic Mechanical Thermal Analysis, Differential Scanning Calorimetry, X-ray diffraction (XRD), scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM). Also the barrier properties against oxygen and water absorption were examined. Results of these experiments showed that additions of 4.96 % catalyst, 7 % activator, 1% polyester polyol and 3% fumed silica produced the best properties with the least crystallinity. KEYWORDS: amorphous, In-situ polymerization, polyamide6, crystallinity, nanocomposites