  We study the supersymmetry enhancement of ABJM theory. Starting from a ${\cal
N}=2$ supersymmetric Chern-Simons matter theory with gauge group
U(2)$\times$U(2) which is a truncated version of the ABJM theory, we find by
using the monopole operator that there is additional ${\cal N}=2$ supersymmetry
related to the gauge group. We show this additional supersymmetry can combine
with ${\cal N}=6$ supersymmetry of the original ABJM theory to an enhanced
  ${\cal N}=8$ SUSY with gauge group U(2)$\times$U(2) in the case $k=1,2$. We
also discuss the supersymmetry enhancement of the ABJM theory with
U($N$)$\times$U($N$) gauge group and find a condition which should be satisfied
by the monopole operator.
