Luminescence properties of Yb-doped Ca-α-SiAlON phosphors with composition of Ca1−xYbxSi12−(m+n)Alm+nOnN16−n were investigated by using cathodoluminescence (CL). The ratio of Yb to Ca was kept constant while the host lattice was changed by replacing m+n(Si–N) bonds with m(Al–N) and n(Al–O) bonds. The luminescence of these phosphors consists of three peaks in the ultraviolet (UV), green (VIS), and infrared (IR) regions, which are attributed to the emissions from secondary phases, Yb2+ and Yb3+, respectively. The UV emission depends on the Si/Al ratio: the UV peak is centered at 310 nm for the Si-rich mix and at 360 nm for the Al-rich mix. We have found that Yb exists in the divalent state in α-SiAlON and in the trivalent state in the secondary phases.