  A molecular rotation assisted non-sequential double ionization (MR-NSDI)
mechanism is identified in the breakup of rotational H$_2$ molecules in a
few-cycle intense laser pulse using a semi-classical trajectory Monte Carlo
method. Applying a molecular source in an appropriate rotational state could
intensively boost NSDI probability, and conclude with an additional small
anti-correlated electron momentum distribution. It reveals the critical role of
the continuous dynamics of molecular rotation, which assists the recollision
process and subsequently releases the excited electron from the potential
portal resulted from molecular rotation. Two underlying exit channels are found
to contribute to MR-NSDI, i.e. tunneling ionization (dominated by enhanced
ionization) and direct ionization. The two channels are confirmed to have
different critical breakup internuclear distances. A prominent nuclear emission
in $(-30^{\circ},30^{\circ})$ along the laser polarized direction is identified
to be a signature of MR-NSDI.
