Method for the strengthening of concrete or timber structures (1, 4), by the application of one or several prestressed Carbon FRP (CFRP) or Glass FRP (GFRP) lamella which are being glued and bonded to the concrete or timber structure (1, 4) using an epoxy resin, wherein the following step is executed a) cutting at least one groove into the concrete or timber structure (1, 4) along the direction in which the concrete structure or timber structure (1, 4) is to be strengthened, and characterized in that the following steps are further executed b) filling the at least one groove tightly with epoxy resin and putting on a layer of epoxy resin onto the section area to be equipped with a CFRP or GFRP lamella, c) putting on a CFRP or GFRP lamella onto the epoxy layer and pressing the same against the concrete or timber structure (1, 4), d) prestressing the CFRP or GFRP lamella between two clamping devices fixed in the concrete or timber structure (1, 4) by a hydraulic jack by pushing the clamp device away from an anchored frame and inserting distance holders between the clamp device and the anchored frame on each end section of the CFRP or GFRP lamella, e) drilling a pair of holes into the concrete or timber structure (1, 4) beside each end section of the CFRP or GFRP lamella, and filling the same with epoxy resin, and putting U-shaped brackets over the two end sections of the CFRP or GFRP lamella by inserting and submerging the both U-legs into the epoxy resin filled opposite holes so the third intermediate U-leg will tightly lay on the CFRP or GFRP lamella, and pressing the intermediate U-leg onto the CFRP or GFRP lamella until the epoxy resin is hardened out, f) after hardening out of the epoxy resin, using the hydraulic jack for again pushing the clamping device away from the frame for removing the spacers, and then carefully releasing the prestress force on both ends of the CFRP or GFRP lamella, and then removing the clamping devices and frames and cutting the end sections of the CFRP or GFRP lamella not glued to the concrete or timber structure or timber structure (1, 4) away. a) cutting at least one groove into the concrete or timber structure (1, 4) along the direction in which the concrete structure or timber structure (1, 4) is to be strengthened, and characterized in that the following steps are further executed b) filling the at least one groove tightly with epoxy resin and putting on a layer of epoxy resin onto the section area to be equipped with a CFRP or GFRP lamella, c) putting on a CFRP or GFRP lamella onto the epoxy layer and pressing the same against the concrete or timber structure (1, 4), d) prestressing the CFRP or GFRP lamella between two clamping devices fixed in the concrete or timber structure (1, 4) by a hydraulic jack by pushing the clamp device away from an anchored frame and inserting distance holders between the clamp device and the anchored frame on each end section of the CFRP or GFRP lamella, e) drilling a pair of holes into the concrete or timber structure (1, 4) beside each end section of the CFRP or GFRP lamella, and filling the same with epoxy resin, and putting U-shaped brackets over the two end sections of the CFRP or GFRP lamella by inserting and submerging the both U-legs into the epoxy resin filled opposite holes so the third intermediate U-leg will tightly lay on the CFRP or GFRP lamella, and pressing the intermediate U-leg onto the CFRP or GFRP lamella until the epoxy resin is hardened out, f) after hardening out of the epoxy resin, using the hydraulic jack for again pushing the clamping device away from the frame for removing the spacers, and then carefully releasing the prestress force on both ends of the CFRP or GFRP lamella, and then removing the clamping devices and frames and cutting the end sections of the CFRP or GFRP lamella not glued to the concrete or timber structure or timber structure (1, 4) away.