  In this paper, we have investigated the correlated electron emission of the
nonsequence double ionization (NSDI) in an intense linearly polarized field.
The theoretical model we employed is the semiclassical rescattering model, the
model atom we used is the helium. We find a significant correlation between
magnitude and direction of the momentum of two emission electrons, and give a
good explanation for this striking phenomenon by observing the classical
collisional trajectories. We argue that this correlation phenomenon is
universal in NSDI process, as revealed by the recent experiment on the argon.
