Imaging of cell nuclei plays a critical role in cancer diagnosis and prognosis. To image noninvasively cell nuclei in vivo without staining, we developed UV photoacoustic microscopy (UV-PAM), in which 266 nm wavelength UV light excites unlabeled DNA and RNA in cell nuclei to produce photoacoustic waves. We applied UV-PAM to ex vivo imaging of cell nuclei in a mouse lip and a mouse small intestine and to in vivo imaging of the cell nuclei in the mouse skin. The UV-PAM images of unstained cell nuclei match the optical micrographs of the histologically stained cell nuclei. Given intrinsic optical contrast and high spatial resolution, in vivo label-free UV-PAM has potential for unique biological and clinical application.