Degradation of electron emission characteristics in single-crystalline and polycrystalline silicon field emitter arrays and its mechanism have been studied. A critical biasing time t(c) at which the emission current started to be degraded dominantly was observed at a pressure of 5 x 10(-7) Torr. The t(c) is shortened as the emission current increases. Also, the emission current repeatedly measured within t(c) with a complete relaxation after each measurement would not be degraded even though the total biasing time exceeded the critical time. The experimental results indicate that the degradation in silicon held emitters mainly originates from thermal instability of the silicon tips, especially a decrease in the effective emitting area rather than changes in the work function and geometrical factor.