Abstract: The dual-wire method is a widely used transverse impedance measurement method. When the transverse impedance of the device under test is very small, large lateral distance between the two wires and large diameter of the wires are prefered to get large signal and thus the measurement results are reliable and reproducible. However, large lateral distance and diameter would lead to large systematic error. A numerical research using HFSS were carried out on the systematic error due to the dual-wire distance and diameter. Its result shows that, for moderately small dual-wire distance and wire diameter, the measured transverse impedance is smaller than the theoretical value calculated by ABCI and HFSS,and larger dual-wire distance and diameter lead to larger systematic error. In the case of a ca