A pre-tensioner for a safety belt of vehicles is provided. The pre-tensioner for a safety belt of a vehicle that is disposed in a side sill of the vehicle and that operates to pull the safety belt by pushing a piston into a tube by a high pressure gas of a gas generator using explosive power of gunpowder that is configured at one side of a bracket in a state in which the piston is connected to the safety belt through a wire within the tube including: internal and external weldnuts that are fixed to each one side of internal and external side surfaces of the side sill, wherein by penetrating the side sill in a vehicle width direction by forming a screw at an external circumference of both end portions of the tube, the both end portions are engaged with the internal and external weldnuts, respectively.