  Using the multi-wavelength data from the Atmospheric Imaging Assembly (AIA)
on board the Solar Dynamics Observatory (SDO) spacecraft, we study a jet
occurred in coronal hole near the northern pole of the Sun. The jet presented
distinct helical upward motion during ejection. By tracking six identified
moving features (MFs) in the jet, we found that the plasma moved at an
approximately constant speed along the jet's axis, meanwhile, they made a
circular motion in the plane transverse to the axis. Inferred from linear and
trigonometric fittings to the axial and transverse heights of the six tracks,
the mean values of axial velocities, transverse velocities, angular speeds,
rotation periods, and rotation radiuses of the jet are 114 km s$^{-1}$, 136 km
s$^{-1}$, 0.81\degr\ s$^{-1}$, 452 s, and 9.8 $\times$ 10$^{3}$ km
respectively. As the MFs rose, the jet width at the corresponding height
increased. For the first time, we derived the height variation of the
longitudinal magnetic field strength in the jet from the assumption of magnetic
flux conservation. Our results indicate that, at the heights of 1 $\times$
10$^{4}$ $\sim$ 7 $\times$ 10$^{4}$ km from jet base, the flux density in the
jet decreased from about 15 to 3 G as a function of
B=0.5(R/R$_{\sun}$-1)$^{-0.84}$ (G). A comparison was made with the other
results in previous studies.
