We present the conceptual design and construction of a mini x-ray computed tomography (mini-CT) scanner designed and built for in vivo small-animal imaging. The imaging system includes an x-ray tube with 0.5 mm focal spot size and a flat panel detector that provides an active area of 41 × 41 cm2 with 200 µm × 200 µm pixel size. The scanner, designed based on the rotating sample idea, features variable imaging magnification and field-of-view (FOV). To this end, the detector and x-ray tube are mounted on linear motion systems, such that the object to detector or object to x-ray source distance can be easily changed. The variable magnification enables to acquire images at different spatial resolution depending on the size of the object being imaged. This design provides a trade-off between the scanner's FOV and imaging spatial resolution in which small objects are imaged at relatively high spatial resolution as smaller FOVs are required. Conversely, larger objects can also be imaged as the scanner's FOV is adjustable at the expense of spatial resolution degradation. The scanner's FOV varies from 43 cm down to 9 cm (in diameter) with a spatial resolution improving from 3.9 cycle/mm up to 14.4 cycle/mm, respectively. This mini-CT scanner is a self-standing versatile imager suitable for various preclinical imaging applications.