In this study, Dy+3, Y+3, and La+3 doped SiAlON ceramics were fabricated by using spark plasma sintering (SPS) technique. The effects of additive cation, starting composition and processing conditions on the morphological evolution as well as on the phase assembly of SiAlON ceramics were investigated. The influence of phase diversity on the optical properties of SiAlON ceramics was also discussed. Results indicate that the translucent α-SiAlON, α/β-SiAlON ceramics and their in-situ composites with AlN-polytypoids could be successfully obtained. The presence of a high amount of secondary phases such as AlN-polytype and the oxynitride glassy phase has been found to reduce the optical transmission of SiAlON ceramics. The doping of Y+3 and Dy+3 cations contributed better to the formation of single-phase α-SiAlON than that of La+3. Doping SiAlON ceramics with Dy+3 cation leads to the formation of relatively homogenised final microstructure. Therefore, higher IR transmission values were observed on Dy-α-SiAlON (42.2%) and Dy-α/β-SiAlON (45.07%) samples.