The present invention relates to a polyethylene terephthalate nanocomposite fiber with enhanced modulus, and specifically to a technique for preparing a PET nanocomposite fiber with excellent initial and high-temperature modulus, which comprises adding 1 to 4% by weight of a compound selected from the group consisting of C 56 H 122 O 12 Si 7 , C 31 H 71 NO 12 Si 8n , C 59 H 127 NO 12 Si 8 , and C 33 H 76 N 2 O 12 Si 8 , each of which is an organic/inorganic hybrid nanocompound, based on the total weight of the polymers, to prepare a polyethylene terephthalate nanocomposite chip having 85 mol% or more of the ethylene terephthalate units and an intrinsic viscosity in the range of 0.50 to 1.20, and then melt-spinning and stretching the composite chip.