  We investigate the significance of isomers on a kilonova associated with
neutron star mergers (NSMs) for the first time. We calculate the evolution of
abundances and energy generation rates ($\dot{\epsilon}_{\rm nuc}$) of ejecta
from NSMs, taking into account $\beta^-$ decay through isomers. We find that
for ejecta with electron fraction ($Y_e$) of $0.2-0.3$, $\dot{\epsilon}_{\rm
nuc}$ is appreciably changed from those without isomers, due to the large
change in timing of $\beta^-$ decay through the isomeric states of ${}^{123,
125, 127} \rm Sn$ and ${}^{128} \rm Sb$. In particular, the effects of the
isomers on $\dot{\epsilon}_{\rm nuc}$ are prominent for ejecta of $Y_e \sim
0.25$, which could emit a fraction of an early, blue component of a kilonova
observed in GW170817. When the excitation from a ground state to its isomeric
state is important, the isomers of ${}^{129} \rm Sb$ and ${}^{129, 131} \rm Te$
also cause additional and appreciable change in $\dot{\epsilon}_{\rm nuc}$.
Furthermore, we demonstrate that larger amounts of lanthanide-free ejecta
result in a better fit of an observed light curve of the kilonova in GW170817
if the isomers are taken into account.
