The self-reinforced Si3N4 ceramics were prepared by pressureless-sintering using ${eta}$-Si3N4 whiskers as a seed. Effects of ${eta}$-Si3N4 whiskers on microstructure and mechanical properties and the ${alpha}$ to ${eta}$ phase transition of Si3N4 were investigated. The self-reinforced Si3N4 ceramics were densified(relative density$geq$98%) by pressureless-sintering (1800$^{circ}C$ 2h) using 8mol% Y2O3 and 6mol% Al2O3 as sintering aids and 5 vol% ${eta}$-Si3N4 whiskers within self-reinforced Si3N4 ceramic seemed to hinder the densification owing to their acicular shapes but accelerated the ${alpha}$ to ${eta}$ phase transition because they acted as pre-existing nuclei. It was found that the more ${eta}$-Si3N4 nucei the faster ${alpha}$ to ${eta}$ phase transition.