We describe a facile room temperature solution-phase chemical approach for large scale synthesis of sub-15 nm ultralong crystalline CuO nanowires with an average diameter of about 8 nm and lengths of up to several tens of micrometers. The results indicated that Cu(OH)₂ nanowires were first formed and subsequently served as template to direct the formation of CuO nanowires. XRD, TEM, SAED and HRTEM were used to systemically investigate the chemical compositions, morphology, size and microstructure features of the as-prepared nanowires. The Raman study of the as-prepared CuO nanowires indicates that the Raman scattering peaks are broadened and red-shifted about 20 cm⁻¹ compared with values of bulk CuO crystals, showing that the as-prepared CuO nanowires exhibit strong quantum size confinement effects. A rational interpretation for the red-shifted and broaden of Raman peaks is given according to phonon confinement model of an infinite crystal. The band gap energy estimated from the UV-vis absorption spectrum of the nanowires was about 3.48 eV, which is apparently much larger than the value of bulk CuO crystals. Thus, UV-vis optical absorption property further confirms the strong quantum size confinement effects of the nanowires. The photocatalytic activity of the as-prepared CuO nanowires was further investigated by evaluating the photo-degradation of a model pollutant rhodamine B (RhB). The results indicated that CuO nanowires have enhanced photocatalytic performance with 97.2% decomposition of RhB after 12 h reaction under UV light irradiation, which was much higher than that of commercial CuO powders (37.6%).