Si and ${alpha}-Si_3N_4$ powder mixtures added with 3 wt% $Y_2O_3$ were reacted under 5 MPa nitrogen pressure. The reaction products contained ${alpha}-Si_3N_4$ particles with elongated shapes. Length and width of the elongated grains were the maximum when the starting powder mixture of 50 wt% Si - 47 wt% ${alpha}-Si_3N_4$ and 3 wt% $Y_2O_3$ was used. Aspect ratio of the elongated grains were between 4.4 and 5. When the starting powder mixture contained 70 wt% Si, large particles with irregular shapes appeared. Meanwhile, the reaction did not proceed when the starting powder mixture contained 30 wt% Si and less. The SHS product was easy to crush and the elongated particles obtained from the starting powder mixtures of 40 wt% Si - 57 wt% ${alpha}-Si_3N_4$ - 3 wt% $Y_2O_3$ and 50 wt% Si - 47 wt% ${alpha}-Si_3N_4$ - 3 wt% $Y_2O_3$ were good candidates for the seeds.