Directional solidification of the zone melting method was performed in order to clarify the growth mechanism on YBa2Cu3Ox (123) superconductors from the partial molten state. The effect of a platinum addition for crystal growth was also investigated. Results indicate that a higher G/R ratio was preferable for the continuous growth of 123 crystals, and lead to the idea that the necessary solute for the peritectic reaction is provided from dispersed Y2BaCuO5 (211) particles in liquid to 123 interface through the liquid. Based on this idea, we developed a solidification model which is in good agreement with the experimental results. In the case of a platinum addition, the coarsening rate of 211 particles decreased, and the cell spacing of 123 crystals also decreased. It is tentatively suggested that the platinum addition decreases the products of the diffusivity in liquid and the Gibbs-Thomson coefficients, which include the surface energy between 211 or 123 phase and liquid,(DLΓ211 and DLΓ123).