Unlike Y-Ba-Cu-O, the Nd-Ba-Cu-O system forms a Nd_1+xBa_2-xCu₃O_7-δ-type solid solution (Nd123ss). An understanding of P_O2 and temperature dependence of the solubility limit will be the key to obtaining high T_c and J_c Nd123ss materials. Therefore, we carefully studied the solubility limit of Nd123ss as a function of P_O2 and temperature. Through X-ray diffraction and differential thermal analysis measurements, we have found that Ba can substitute for Nd site under low P_O2 at high temperatures. Furthermore, DC magnetization measurements showed that the superconducting transition temperature of Ba-rich Nd123ss (x<0) is higher than that of Nd rich Nd123ss (x≥0).