Nonequilibrium Greens function method in a tight-binding approximation is used to calculate current through a carbon nanotube field-effect transistor (CNFET) with cylindrical gate electrode. Two individual defects in gate oxide (SiO₂) of a long channel p-type CNFET is investigated. It is found that the relative current reduction and the threshold voltage shift caused by two charged defects depend sensitively on positions of the charged defects. If the charges Q₁=Q₂=+e (where -e is electron charge) are both close to one of the two leads, or one is close to the source lead and the other is close to the drain lead, the relative current reduction in on-state is much greater than that due to a single positive charge Q=+e located close to one of the two leads. In this case, the threshold voltage shift due to two charges is negligible. If two positive charges are both around center of the channel, the relative current reduction in on-state caused by two charges is significantly greater than that caused by a single positive charge near center of the channel. Furthermore, the negative shift of threshold gate voltage increases with decreasing distance between two charges. And a great threshold voltage shift of -0.35 V may be caused.