  In this paper we will analyse the ABJM theory in harmonic superspace. The
harmonic superspace variables will be parameterized by the coset $SU(2)/U(1)$
and thus will have manifest $\mathcal{N} =3$ supersymmetry. We will study the
quantum gauge transformations and the BRST transformations of this theory in
gaugeon formalism. We will use this BRST symmetry to project out the physical
sub-space from the total Hilbert space. We will also show that the evolution of
the $\mathcal{S}$-matrix is unitary for this ABJM theory in harmonic
superspace.
