Stereotactic body radiation therapy (SBRT) is a promising modality in the treatment of cancer. Despite success in early clinical applications and in subsequent clinical trials, there remains much to learn to understand and optimize the effects in tumors and normal tissues. In this study, we develop and characterize an image-guided small animal stereotactic body irradiator capable of delivering a highly localized radiation beam accurately to a small target. The irradiator employs a commercial X-ray device (XRAD 320, Precision X-Ray, Inc.) and a modular collimation system, consisting of a brass collimator holder and variable tungsten alloy collimators with apertures ranging from 1 to 10 mm in diameter. The unit is typically operated at 30 kVp for image guidance and at 250 kVp for therapy. To characterize the radiation beam, we measured percent depth dose (PDD), off-axis ratios (OARs), and absolute dose rate for each collimator using radiochromic film (Gafchromic EBT, International Specialty Products, Wayne, NJ). For all collimators the penumbra, defined as distance between 80% and 20% isodoses, was measured at a source-to-surface distance (SSD) of approximately 20 cm. For a 5 mm collimator, in-plane and cross-plane measurements of penumbra were 0.7 and 1.1 mm, respectively. Absolute dose rates ranged from 6.7 to 11.6 Gy/min for the 1 to 10 mm collimators. The image guidance system provided accuracy commensurate with stereotactic localization in small animals. We have demonstrated the capability of an image-guided stereotactic radiotherapy by the animal studies. Subsequent studies in a variety of pre-clinical tumor and normal tissue models suggest that use of the irradiator can significantly contribute to the understanding of new clinical therapies such as SBRT.