YBa 2Cu 3O x (Y123) films can not be grown on MgO substrates by liquid phase epitaxy (LPE) directly, but they have been grown successfully on the substrates with Y123 thin films (seed films) deposited from a vapor phase. We employed a partial vertical dipping to investigate the initial growth mechanism of the Y123 films on (100)MgO substrates with the seed films. At the initial stage of LPE, the seed films partially dissolved in the BaOCuO solution, and island crystallites epitaxially formed on the MgO substrates. The results indicate that the grains in the seed films which exceed the critical grain size serve as nuclei, and the smaller grains dissolve. The island crystallites extended laterally and coalesced with one another. Subsequently, spiral growth proceeded, and finally the films grew to be single-crystalline.