Nitrogen ions implanted into the buried oxide layer can increase the total dose radiation hardness of silicon on insulator (SOI) materials. However, the obvious increase in positive charge density in the buried layer with high dose of nitrogen implantation leads to a negative effect on the technology of nitrogen implantation into buried oxide. In order to suppress the increase in positive charge density in the nitrogen-implanted buried layer, co-implantation of nitrogen and fluorine is used to implant fluorine into the nitrogen-implanted buried layer. High-frequency voltage-capacitance (C-V) technique is used to characterize the positive charge density in the buried layer. Results show that, in most cases, using the co-implantation of nitrogen and fluorine can significantly reduce the positive charge density in the nitrogen-implanted buried layer. At the same time, it is also found that further increase of the positive charge density induced by fluorine implantation in the nitrogen-implanted buried layer can occur in particular cases. It is proposed that the decrease in the positive charge density in the fluorine and nitrogen-implanted buried layer is due to the introduction of electron traps into the buried layer through fluorine implantation.