We used the method of moments (MoM), which is a numerical electromagnetic field computation method, to study the surge characteristics of twisted pair single-phase electric power lines. The results show that wires stranded with less than several turns per meter have almost the same surge characteristics as parallel lines, but the surge impedance decreases in tens of % when the number of strands increases. To verify the simulation results, we measured the surge characteristics of a model line of twisted pair experimentally, and there is good accordance between them. It is possible to simulate the Surge in twisted pair covered with polymer dielectric insulators by MoM. Moreover, we studied the surge characteristics of twisted pair which is inserted into a metal tube. According to the results, the effect of the strand is smaller than that in free space without [he metal pipe. There are two propagation modes in common mode: fast surge and slow surge. Few influences of the number of strands on the surge velocity occur in the fast propagation surge, but the increase of the surge impedance and the slowdown of the propagation speed are caused by twining in the slow propagation mode. (C) 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(3): 1-9, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20737