Titanium dioxide (TiO 2 ) nanosized powder was synthesized by pulsed wire discharge (PWD) using Ti wire in O 2 gas. The pressure of O 2 gas (P) and relative energy (K), which is defined as the ratio of the charged energy of the capacitor to the vaporization energy in the wire, were varied from 10 to 100kPa and 4.3–51.9, respectively. The density of the plasma/vapor (D exp ), determined by the maximum volume, was decreased with decreasing P and increasing K as observed by high-speed photography during PWD. X-ray diffraction analysis confirmed that the rutile content (C R ) was increased from 7 to 84vol% with decreasing D exp . The median diameter (D 50 ), determined from scanning electron microscopy, was decreased from 36.5 to 10nm with decreasing D exp . Therefore, an increase in C R and decrease in D 50 both accompany the decrease in D exp .