  We extend the even weight modular forms of modular invariant approach to
general integral weight modular forms. We find that the modular forms of
integral weights and level $N$ can be arranged into irreducible representations
of the homogeneous finite modular group $\Gamma'_N$ which is the double
covering of $\Gamma_N$. The lowest weight 1 modular forms of level 3 are
constructed in terms of Dedekind eta-function, and they transform as a doublet
of $\Gamma'_3 \cong T'$. The modular forms of weights 2, 3, 4, 5 and 6 are
presented. We build a model of lepton masses and mixing based on $T'$ modular
symmetry.
