This study demonstrates a technique to measure electrical resistivity of the calf in hemodialysis (HD) patients during HD treatment. To continuously monitor and calculate resistivity, a model of calf volume based on its geometrical size and measurement of its electrical resistance has been developed. The model makes it possible to continuously estimate reduction of the calf circumference during HD. Seventeen HD patients were studied during HD using a multi-frequency bioimpedance device (Xitron 4200). Circumference of the calf was measured by a measuring tape pre- and post- HD for each treatment. Results showed a high correlation between measurement and calculation circumference in post HD (r2=0.985). Further, the value of resistivity normalized by body mass index (BMI) provides information about patients' hydration state in comparison to those in healthy subjects. This technique is useful for identifying the range of optimal hydration states for HD patients.