The carbon quantum dots (CQDs) were prepared by using an easy and economic onepot method. The starting materials are the ascorbic acid as a carbon source and copper acetate as a catalyst to accelerate the reaction. The structural and optical properties of the CQDs are studied by UV-VIS spectrophotometer and photoluminescence (PL) measurements, transmission electron microscope (TEM), X-ray diffraction (XRD) and Fourier transform-infrared spectroscopy (FTIR). The excitation wavelength and the pH-dependent of PL properties was investigated also. The results show that the CQDs have strong absorption peak in the UV-region. The PL is the excitation wavelength dependent. The PL of CQDs decreases upon lowering the pH value of the solutions and the PL peaks shifted to the shorter wavelength. The average size of the CQDs is 4 nm from the TEM measurements.