Stable ethylene glycol (EG)-based nanofluids containing graphene nanoplatelets (GnPs) were prepared by intensive ultrasonication without any surfactant. The structural properties of the commercially produced GnPs were confirmed using the nitrogen gas adsorption method, Fourier transform infrared spectroscopy, X-ray diffraction method, Raman spectroscopy, and high-resolution transmission electron microscopy. After ultrasound irradiation, the GnP aggregates were broken into thinner and smaller-sized nanosheets, which is beneficial for a stable dispersion. The ultrasonic-treated GnPs showed a constant value of thermal conductivity enhancement, k/k(o) (= 1.127 +/- 0.002) at 2 vol% in the temperature range of 10-90 degrees C. From the analyses of the thermal conductivities of the GnP nanofluids as functions of GnP concentration and temperature, it was concluded that the thermal conductivity increased as the GnP concentration and the temperature increased. Furthermore, the experimentally measured thermal conductivities of the EG-based GnP nanofluids were much higher than the theoretically calculated values based on the Hamilton-Crosser correlation, which is due to higher specific surface area and two-dimensional structures of the GnPs.