Application of transverse magnetic field (TMF) is one of the most important ways to improve electric life and breaking capacity of DC relays. For better understanding of dependence of arc durations on transverse magnetic field, a series of experiments were conducted under an external transverse magnetic field with 12 pairs of AgSnO₂ contacts in a DC 28V 60A/30A/5A circuit, respectively. By using permanent magnets, the transverse magnetic field was obtained and the magnetic flux density at the gap center was varied from 13 to 94mT. The results show that breaking arc duration is decreased monotonically with increases in the magnetic flux density, but making arc duration isn't decreased monotonically with increases in the magnetic flux density. In addition, both the magnetic flux density and the breaking arc duration have threshold values B_l and T_bmin, respectively, which means the breaking arc duration is almost stable with the value T_bmin even if the magnetic flux density is higher than B_l.