Urchin-like CuO, consisting of closely packed nanorods with a diameter of 10nm, have been successfully synthesized by a poly(ethylene glycol) (PEG)-assisted hydrothermal route at low temperature of 100°C. The as-obtained Urchin-like CuO were thoroughly characterized by X-ray diffraction (XRD) study, Field emission scanning electron microscope (FESEM), High-resolution transmission electron microscopy (HRTEM) and Gas sensor measurements. From the XRD pattern, all the peaks detected can be assigned to CuO in a monoclinic structure with lattice parameters a=4.662, b=3.416 and c=5.118 (JCPDS card no. 65-2309). The FESEM and TEM showed that the diameter of the urchin-like CuO sphere is about 1µm. Further investigation of the formation mechanism reveals that the PEG-assisted hydrothermal process is vital to the formation of 3D structures. Besides the template function, PEG often plays as a reductant while reacting with Cu(+2). In our case, no impurity peaks of Cu2O were observed in the XRD pattern, implying that PEG did not reduce Cu(+2) to Cu(+1). We attribute this to the high concentration of PEG. The sensor based on the urchin-like CuO nanostructures exhibit excellent ethanol-sensing properties at reduced working temperature (200°C), which shows a sensitivity two times higher than that of CuO particles(about 100nm, made from calcinations of Cu(NO3)2 at 400°C). The enhancement in sensitivity of the as-prepared CuO may be contributed to the fancy 3D nanostructures.