Abstract: A safety device for on/off switch of an electric tool comprises a casing having a switch and a link assembly, in which the link assembly is composed of a knob, a first link lever, a second link lever, a trigger, and a dog. The pivotal joint correlation among those components are: the trigger, the second link lever, and the knob to the casing; the first link lever to the trigger; the second link lever to the first link lever; the knob to the second link lever; and a dog to the first link lever. Moreover, a protruding portion thereof is positioned corresponding to the switch. In operation of an electric tool of this kind, a user is requested to depress both said trigger and knob with his two hands respectively and simultaneously so that the dog is pulled upwardly to travel a longer-distance stroke to have its protruding portion depressed the switch to trigger the power, otherwise, by depressing either said trigger or knob with a single hand, the operation is in vain.