Single-phase (Ca,Dy)-α-Sialon with elongated crystals and high aspect ratios was prepared at 1700°C by carbothermal reduction and nitridation (CRN) with Y-α-Sialon seed crystals. Effects of Y-α-Sialon seeding and holding time on the phase assemblage and crystal morphology of CRN products were discussed. The results showed that adding seed crystals and prolonging holding time could facilitate not only phase transformation, but also crystal refinement. Moreover, they resulted in the increase of the aspect ratios of α-Sialon crystals, due to the decreasing range in the crystal width exceeding that in the length. In addition, a highly unusual phenomenon of crystal size reduction during reaction was observed in the seeded samples with the holding time extending from 2 to 4 h. Crystallization from liquid phase and growth followed by nucleation based on the surface of existing large crystals were the growth modes of elongated α-Sialon crystals.