We have studied the quasi-binary phase diagram along the line of NdBa₂Cu₃O_y and BaCuO₂ in 1%O₂+Ar. In this study, the nucleation and melt of NdBa₂Cu₃O_y in Ba-rich solvent were observed in situ using a laser-scanning microscope under 1%O₂+Ar. The liquidus and solidus lines of nearly stoichiometric Nd123 were constructed. Phase identification has been performed on the samples quenched from the reaction temperatures determined by differential thermal analyses through X-ray powder diffraction measurements and scanning electron microscopy.