Rare-earth-doped Ca-alpha-SiAlON phosphors, with the compositions of (Ca1-3/2xREx)(m/2)Si12-m-nAlm+nOnN16-n (RE=Ce, Sm, and Dy, 0.5 <= m=2n <= 3.0), were prepared by sintering at 1700 degrees C for 2 h under 10 atm N-2. The concentration of rare earths varied from 3 to 30 at.% with respect to Ca. The photoluminescence (PL) properties were investigated as functions of the composition of the host matrix (i.e., m) and the concentration of rare earths (i.e., x). The results show that the emission properties can be optimized by tailoring m and x. The Ce3+ luminescence originating from the 4f-5d interconfigurational transitions is greatly affected by the environment surrounding the Ce3+ ions, which differs from the Sm3+ or Dy3+ luminescence arising from the 4f-4f intraconfigurational transitions. X-ray diffraction and scanning electron microscopy were used to explain the composition and concentration dependence of PL properties.