Pure Cadmium Oxide (CdO) and 3d transition ion doped CdO were prepared by co - precipitation method. CdO was doped with cobalt, nickel, iron, manganese and chromium. The pure and doped CdO were pelletized. With these pellets, ultrasonic velocity was determined using pulse echo method. Density measurements were done by Archimedes principle. These measurements were used to calculate Debye temperature. Theoretical investigation was carried out to find the variation in vibrational energy and Debye cut-off frequency of CdO due to doping.