Figure 2 shows the XRD patterns of the films prepared using various Cu-sol concentrations heated to 200 °C. The CuO diffraction pattern was a well-ordered monoclinic structure (JCPDS no.: 80–1268). Increasing the Cu concentration of the prepared sol increased the crystalline size from 5.6 (0.2 mol % Cu) to 7.6 nm (0.4 mol % Cu), as shown in Table 1. The intensities of the crystalline phases also increased with increasing Cu concentration, due to the increase in the FWHM of the main peak. The crystalline sizes and crystallinity of the CuO thin films were strongly related to the initial copper concentration. The increase in crystalline size with increasing Cu-sol concentration can be attributed to atomic diffusion and agglomeration of the Cu in the CuO thin film during the heating process at 200 °C. In addition, we confirmed that when the Cu-sol concentration was increased from 0.2 to 0.4 mol %, the interplanar spacing of the deposited CuO films was changed from 0.237 to 0.259 Å, and the diffraction plane of the main peak was changed from (111) to (−111). We attribute this to the fact that the interplanar spacing of Cu-sol widens with increasing CuO crystal size when the initial Cu-sol concentration is increased.