We report on the growth of CdO and Cd(OH) 2 nanostructures with different morphologies on Si substrates by using a hydrothermal method. Structural and optical investigations showed that the shape and phase could be modified through the growth time and the concentration in the solution. The formation of CdO was dominated during the early stage of growth and at a low source concentration, while that of Cd(OH) 2 was dominated at the later stage of growth with a higher source molar concentration. And the mechanism was explained by the thermodynamically favorable reaction. Our results indicate that the morphology and crystalline phases of CdO and Cd(OH) 2 nanostructures can be controlled on the Si substrates for various applications.