The preparation of $eta$-SiAlON powder by SHS in the system of $Si-Al-SiO_2-NH_4F(eta-Si_3N_4)$ was investigated in this study. In the preparation of SiAlON powder, the effect of gas pressure, compositions such as Si, $NH_4F$, eta-Si_3N_4$ and additive in mixture on the reactivity were investigated. At 50 atm of the initial inert gas pressure in reactor, the optimum composition for the preparation of pure $eta$-SiAlON was $3Si+Al+2SiO_2+NH_4F$. The $eta$-SiAlON powder synthesized in this condition was a single phase $eta$-SiAlON with a rod like morphology.