  Using first principle approaches, we investigate the effects of isotope
substitution on the inelastic features in the hydrogen molecular junction. We
observe thatlocal heating and inelastic current have significant
isotope-substitution effects. Due to the contact characters, the energies of
excited molecular vibrationsare inverse proportional to the square root of the
mass. The heavier the molecule, the smaller the onset bias. In the $H_{2}$ and
$D_{2}$ junctions, the heavier molecule has a smaller magnitude of
electron-vibration interaction. Consequently, there is a crossing in the local
temperature around $80 K$. In the HD junction, the electron-vibration
interaction is enhanced by asymmetric distribution in mass. It leads to the
largest discontinuity in the differential conductance and the most prominent
heating in the HD junction. We predict that the junction instability is
relevant to isotope substitution. The HD junction has the smallest breakdown
voltage compared with the $H_{2}$ and $D_{2}$ junction.
