An internal calibration mechanism of a weigh module has a driving structure, a calibration weight and a weight support frame. The weight support frame has an opening or a groove for loading the calibration weight. The weight support frame is connected to a load receiving portion at both sides thereof and is connected to a portion of a fixing portion that extends towards the load-receiving portion. In one case, the weight support frame, the load-receiving portion and the fixing portion are integrally formed. In another case, a force transmission connecting portion and a fulcrum connecting portion of the weight support frame are fixedly connected, respectively, to the load-receiving portion and to the extending portion of the fixing portion. In another case, flexure hinges connect the weight support frame to the load-receiving portion at both sides thereof and connect the weight support frame to the extending portion.