If the velocity of a charge particle is perpendicular to the B field,
The force q vXB will cause a particle to move in a circle with constant velocity.
From qvB=m v[sup]2[/sup]/r so r=mv/(qB)
The E field will cause a particle to accelerate along the field direction.

Now, we have a charge particle in E and B field.
The circular motion radius r=mv/(qB), which is dependent on the velocity of the particle.
Because the velocity is changing due to E field, so the circular radius r also is changing,too.
The changing in radius cause the particle to drift in the EXB direction, so called EXB drift (E cross B drift).