ABSTRACT. We examined the utility of a bite block-type head immobilization device, hereafter referred to as "head immobilization device", in order to improve the ease of immobilization and accuracy when performing radiotherapy for cranial tumors in animals. The head immobilization apparatus was a rectangular-shaped bite block-type device. We examined 55 cases in 46 dogs that underwent head CT scans between June 2005 and May 2006. The head immobilization device was used for 26 cases (immobilization group) and was not used for 29 cases (control group). Head stability was maintained in the control group by placing a towel under the head. We measured the angle of rotation of the xy, yz and xz planes for each group. The angles of rotation of the xy plane for the control and immobilization groups were 3.69  2.28 (mean  SD) and 1.39  1.50, respectively. The t-test demonstrated that the difference was statistically significant (p<0.001). These results indicate that there was reduced tilting to the left or right. We conclude that use of this head immobilization device was extremely easy and that it improved the accuracy of radiotherapy for cranial tumors in dogs and cats.