Fluorine-doped SiO2(FSG) has a low dielectric constant just after deposition. However, the dielectric constant of FSG gradually increases with time when left in air. We have developed a way to stabilize dielectric constants of FSG left in the air by plasma annealing. We have found that N2O-plasma annealing is quite effective for blocking moisture. The dielectric constants of FSG treated by the N2O-plasma annealing rarely change.