  Understanding the jitter noise resulting from single-pulse phase and shape
variations is important for the detection of gravitational waves using pulsar
timing array. We presented measurements of jitter noise and single-pulse
variability of 12 millisecond pulsars that are part of the International Pulsar
Timing Array sample using the Five-hundred-meter Aperture Spherical radio
Telescope (FAST). We found that the levels of jitter noise can vary
dramatically among pulsars. A moderate correlation with a correlation
coefficient of 0.57 between jitter noise and pulse width is detected. To
mitigate jitter noise, we performed matrix template matching using all four
Stokes parameters. Our results revealed a reduction in jitter noise ranging
from 6.7\% to 39.6\%. By performing longitude-resolved fluctuation spectrum
analysis, we identified periodic intensity modulations in 10 pulsars. In PSR
J0030+0451, we detected single-pulses with energies more than 10 times the
average pulse energy, suggesting the presence of giant pulses. We also observed
a periodic mode-changing phenomenon in PSR J0030+0451. We examined the
achievable timing precision by selecting a sub-set of pulses with a specific
range of peak intensity, but no significant improvement in timing precision is
achievable.
