Abstract: The moment of the resultant, equivalent force about a given point of a force system, is equal to the sum of the moment of all acting forces about the same point. This theorem is usually used to determine the center of a parallel force system. In a conventional way the acting point of the resultant is set to be the center, which is difficult to be understood. In this work, an arbitrary point on the acting line of the resultant is assumed in using the theorem. The physical meaning of the center is clear and the line of action of the resultant must always pass through a fixed point.