The bond interface of a TLP (Transient Liquid Phase) bonded tin has been observed with a TEM to investigate the effect of the liquid phase on the behavior of the superficial oxide film at the interface during the bonding process. In the solid-state-diffusion bonded joint without filler, abundant oxide inclusions were observed to be distributed within a region of a few 100 nm widths along the bond interface. In comparison with this, the liquid phase introduced by the eutectic reaction of the bismuth filler with the tin substrate decreased the width of the interfacial region involving abundant oxide inclusions to form a rather layer structure a few 100 nm thick consisting mainly of SnO2. It also enhanced the annihilation of the uncontacted areas at the interface. The layer of the oxide became discontinuous and coalesced with an increase in bonding temperature and pressure, and areas where no oxide inclusion could be observed at the interface were increased, when the liquid phase was formed. Owing to these effects, the bond strength rose at lower bonding temperatures and pressures when the bismuth filler was applied.