In order to optimize the design of undulators using high-temperature superconductor (HTS) bulks we have developed a method estimate critical current density (${J}_{c}$) each bulk from overall measured magnetic field an undulator. The vertical was along electron-beam axis in HTS bulk-based undulator consisting twenty Gd-Ba-Cu-O (GdBCO) inserted 12-T solenoid. ${J}_{c}$ values were estimated by i...