A series of cadmium oxide samples (CdOs) were synthesized by thermal treatment method with various annealing temperatures (TA = 500 ℃ - 900 ℃). All samples crystallize in a pure cubic structure with Fm̅m symmetry. The crystallite size, lattice parameter and volume change slightly with TA, and show an unexpected inflection point for the sample with TA = 800 ℃ (CdO-800), while the average particle size increases monotonically from ∼0.4 µm to ∼3.0 µm. Distinct ferromagnetic features accompanied by diamagnetic and/or paramagnetic phases were detected. The XPS data indicate the valence of cadmium and oxygen vacancy concentration varies with TA, which are responsible for the ferromagnetic origin in CdOs. CdO-800 shows the maximum magnetization, significant coercive force and residual magnetization. Oxygen vacancy plays a crucial role in the ferromagnetic performance, which can be effectively controlled by changing the sintering temperature. This work claims new characteristics for CdO besides optical and semi-conducting, which may be important in the preparation and possible applications for CdO dilute magnetic semiconductors.