Effect of the trapped Si ions in a 30-nm gate oxide implanted with Si+ at a very lowener gy (1.3 keV) on the flatband voltage after various thermal annealing has been examined. For the annealing at 700 .C for 20 min, although only 0.1% of the implanted Si ions remained, it can cause a flatband voltage shift of .21.3 V, and the flatband voltage shift reduces with time under a negative gate voltage showing neutralization of the trapped ions by the injected electrons from the gate. However, the annealing at 900 .C for 20 min has reduced the number of the remaining ions to the lowest limit corresponding to a flatband voltage shift of .0.1 V, and the application of the negative voltage does not change the flatband voltage. A higher annealing temperature or a longer annealing time does not showfurther improvement, suggesting that the annealing at 900 .C for 20 min is sufficient for eliminating the effect of the trapped ions.