This article deals with the effects of the contact ratio ε on transmission errors of trochoidal gears (which consist of a roller gear anda cam gear). First, the experiments and multibody analysis (MBA) for the transmission errors of two types of single-row trochoidal gears (types A and B gears) were carried out. The type A gear is a commercial trochoidal gear with ε = 1.1 and the type B gear is a trochoidal gear with ε = 2.1 (by increasing the number of teeth). The experimental and MBA results showed that the peak-to-peak value TEP-P of the transmission errors of the type B gear (with ε = 2.1) was lower than the type A gear (with ε = 1.1). The TEP-P of types A and B gears increased as the rotational speed of the roller gear increased. However, the increasing rate of the measured TEP-P of the type B gear due to an increase of the rotational speed was less than that of the type A gear. Increasing the contact ratio due to an increase in the number of teeth in a single-row trochoidal gear (such as a type B gear) decreases the strength of the teeth and rollers. To overcome this problem, as a new transmission error reduction method, a double-row trochoidal gear (type C gear), having two times the contact ratio of the type A single-row trochoidal gear was presented and its transmission error was examined. The experimental and MBA results showed that the TEP-P of the transmission errors of the type C double-row trochoidal gear were lower than that of the type A single-row trochoidal gear. Therefore, it is clear that using a double-row trochoidal gear is effective for reducing the transmission errors of trochoidal gears.