A switching mode power supply and a switch thereof are provided. The switch includes a plurality of first transistors, and a second transistor that is turned on/off by a control signal that is equal to that of the plurality of first transistors and in which a second current corresponding to a first current flowing to the plurality of first transistors flows, wherein a ratio of the first current to the second current sequentially changes from a time point at which the second transistor is turned on. Therefore, a switching mode power supply and a switch thereof that can always uniformly sustain a maximum limit current flowing to the switch regardless of a level of an input voltage without including a special additional circuit can be provided.