Graphene nanosheets have been prepared from low-temperature expandable graphite by ball milling and subsequent ultrasonication. The as-prepared graphene nanosheets were characterized by ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, transmission electron microscopy, atomic force microscopy, differential thermal analysis, etc. The graphene concentration in the suspensions could be increased proportionally by simply extending the milling and ultrasonication duration. The graphene morphology changed from ribbons to single- and few-layer nanosheets with a porous nature. After milling for 40 h, the average number of stacking layers was estimated to be 3.5 for the graphene nanosheets. The lateral dimension of the defect-free crystallites decreased with the milling time, whereas the structural defects mainly from the graphene edges increased accordingly. The exfoliation process was interpreted in terms of complex factors, including weak interlayer attraction, shear-induced dislocation bands, and acoustic cavitation of ultrasonic irradiation. The sheet resistance of the graphene films dramatically decreased after the thermal treatments, especially in reducing atmospheres. This facile approach appears promising for its potential applications in functional composite materials.