The authors developed a phantom, designated as the CAGN phantom, compatible for MRI and hyperthermia that is useful in the fundamental studies of non-invasive MR thermometry. The ingredients of this phantom are carrageenan, GdCl₃ as a T₁ modifier, agarose as a T₂ modifier, NaCl as a conductivity modifier, NaN₃ as an antiseptic and distilled water. Another phantom that was developed, the CAG phantom, has relaxation times that are adjustable to those of any human tissue. To use this phantom for electromagnetic heating, NaCl was added to change the conductivity of the phantom and clarified the relationship between the conductivity and NaCl concentration. This study examined the relationship between relaxation times and NaCl concentration of the CAGN phantom. The results showed that both T₁ and T₂ values were affected by NaCl and the experimental results led to the empirical formulae expressing the relationship between the relaxation rates (1/T₁, 1/T₂) and the concentrations of GdCl_3, agarose and NaCl. The appropriate concentrations of T₁ and T₂ modifiers were calculated from these empirical formulae when making a specified phantom that has the required relaxation times and NaCl concentration.