Recently, we reported an animal semiconductor PET with the spatial resolution of 0.8mm FWHM within the central 20mm-diameter of FOV for the purpose of biomedical study using rats and mice. This ultra high spatial resolution was obtained by the use of small CdTe elements of 1.1mm x 1.0mm x 5mm. The FOV of this PET is 64mm in diameter and 26 mm in axis. We applied to observe small tumors in mouse and succeeded to obtain [F-18]FDG images of mouse mammary carcinoma of similar to 1mm size.