The melting temperature of Fe-18 wt% Si alloy was determined up to 119 GPa based on a change of laser heating efficiency and the texture of the recovered samples in the laser-heated diamond anvil cell experiments. We have also investigated the subsolidus phase relations of Fe-18 wt% Si alloy by the in-situ X-ray diffraction method and confirmed that the bcc phase is stable at least up to 57 GPa and high temperature. The melting curve of the alloy was fitted by the Simon's equation, P(GPa)/a = (T (m)(K)/T (0)) (c) , with parameters, T (0) = 1,473 K, a = 3.5 +/- A 1.1 GPa, and c = 4.5 +/- A 0.4. The melting temperature of bcc Fe-18 wt% Si alloy is comparable with that of pure iron in the pressure range of this work. The melting temperature of Fe-18 wt% Si alloy is estimated to be 3,300-3,500 K at 135 GPa, and 4,000-4,200 K at around 330 GPa, which may provide the lower bound of the temperatures at the core-mantle boundary and the inner core-outer core boundary if the light element in the core is silicon.