Magnetic field stability and hysteresis can be serious problems in a high-field high-temperature superconducting magnet, because of the large shielding current effect. Accordingly, the field stability of a 25 T cryogen-free superconducting magnet (25T-CSM) was investigated. This particular system can generate a central field of 24.6 T in a 52-mm room-temperature bore. It was found that the magnetic field relaxation rate of the 25T-CSM with a Bi2Sr2Ca2Cu3Oy (Bi2223) insert was due to the flux creep effect, and was small enough to allow nuclear magnetic resonance (NMR) experiments. Based on these experiences in the development of the 25T-CSM, primitive designs for a 30 T cryogen-free superconducting magnet (30T-CSM) were suggested based on upgrading the present Bi2223 insert to a REBa2Cu3Oy (RE123, RE: rare earth and Yttrium) insert incorporating two co-wound RE123 tapes. It was confirmed that the prototype test coil could be operated at 4.2 K under large electromagnetic stresses of up to 540 MPa without experiencing degradation.