[Objective]We assessed whether ionized calcium (iCa), total Ca (tCa), or serum albumin level (Alb)‒corrected Ca is optimal for evaluating Ca levels in hemodialysis (HD) patients.[Method]We examined the relationships between pH‒corrected iCa (pH‒iCa) according to gas analysis and tCa, Alb‒corrected Ca based on the K/DOQI‒1 formula (KDOQI‒Ca), or Alb‒corrected Ca based on the Ca ionization rate (CaIR) obtained by dividing the pH‒corrected iCa by tCa; i.e., the CaIR‒Ca, before and after HD in 43 patients.[Results]Before and after HD, the relationship between the CaIR and Alb was y=-0.011x+0.558 (r=0.199, p>0.2) and y=-0.031x+0.655 (r=0.720, p<0.0001), respectively. Before HD, the relationships between pH‒iCa and tCa, KDOQI‒Ca, or CaIR‒Ca were y=7.73x‒0.01 (r=0.862), y=7.46x+0.30 (r=0.846), and y=7.61x+0.21 (r=0.859), respectively. After HD, these relationships were y=3.50x+5.08 (r=0.482, p<0.005), y=3.81x+4.66 (r=0.460, p<0.002), and y =1.55x+7.36 (r=0.282, p<0.1), respectively. The attenuation of the relationship between pH‒iCa and tCa after HD seems to have occurred because the increase in the Ca‒bound Alb concentration caused by HD hindered the transfer of iCa from the dialysate to plasma.[Conclusion]For evaluating the Ca levels of HD patients, iCa, tCa, KDOQI‒Ca, and CaIR‒Ca can be used before HD, but iCa should not be used after HD.