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We present our results of the effects of coplanar electrode's geometrical factors on the measurement of bioelectrical impedance, one of the most widely measured physical properties in biomedical applications. Coplanar electrodes of different shapes and sizes were used to measure the electrical impedance of saline at different concentration and blood reconstituted at different hematocrit in either plasma or serum. The results show that the impedance measurement by coplanar electrodes are sensitive to the geometry of the electrode as well as the sample boundary in addition to the universal determinants such as the ionic or cellular concentration of the material under test. This implies that all the geometrical parameters of a coplanar electrode ought to be taken into account collectively of its design when it is used in biomedical instrumentation. Thus the reliability of the instrument as well as the measurement may be assured.