  We reexamine whether $Z^+(4430)$ could be a $D_1'-{D}^*$ or $D_1-{D}^*$
molecular state after considering both the pion and $\sigma$ meson exchange
potentials and introducing the form factor to take into account the structure
effect of the interaction vertex. Our numerical analysis with Matlab package
MATSLISE indicates the contribution from the sigma meson exchange is small for
the $D_1'-{D}^*$ system and significant for the $D_1-{D}^*$ system. The S-wave
$D_1-\bar{D}^*$ molecular state with only $J^{P}=0^-$ and $D_1'-{D}^*$
molecular states with $J^P=0^-,1^-,2^-$ may exist with reasonable parameters.
One should investigate whether the broad width of $D_1'$ disfavors the possible
formation of molecular states in the future. The bottom analog $Z_B$ of
$Z^+(4430)$ has a larger binding energy, which may be searched at Tevatron and
LHC. Experimental measurement of the quantum number of $Z^+(4430)$ may help
uncover its underlying structure.
