The mechanism of improving the in-plane alignment of LPE-grown YBCO films on MgO substrate was investigated by the experiments and model analysis. The preferential dissolution and growth of the islands in seed film due to the difference in the in-plane alignment relationship between the island and substrate were observed even when the size of the seed islands were similar. The phenomena can be explained by the coarsening mechanism by considering the difference in the interface energies between YBCO grains and MgO substrate. The preferential growth model was developed based on the above idea. The results of the calculation by the model are in good agreement with the experimental results.