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 (ǫ nuc ) of ejecta from NSMs, taking into account β − decay through isomers. We find that for ejecta with electron fraction (Y e ) of 0.2 − 0.3,ǫ nuc is appreciably changed from those without isomers, due to the large change in timing of β − decay through the isomeric states of 123,125,127 Sn and 128 Sb. In particular, the effects of the isomers onǫ nuc are prominent for ejecta of Y e ∼ 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 Sb and 129,131 Te also cause additional and appreciable change inǫ 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.