The method for manufacturing a stacked triangular transformer having three legs including stacked laminations, wherein a cross-sectional plane of each leg has two leg halves and having six yoke segments, making use of an assembly stand with a base plate, fixture adapters and a securing bar, where the method comprising a step of assembling a triangular core, releasing clamping beams (6,7) and yoke segments at the top of the triangular core and taking them temporary out of the transformer core, positioning the coil windings on three opened core legs, assembling the top yoke segments and tightening them by top clamping beams (6,7) and securing means and placing the transformer core onto a transformer tank, characterized in that the step of assembling a triangular core comprises the following steps: a) positioning of two halves of one leg on the base plate of the assembly stand on the horizontal position using a lower part of adapters for maintain the proper angle between the two halves of the one leg, b) securing the positioned halves using an upper part of adapters and the securing bar placed on steel rods, c) positioning of an outer clamping beam underneath two leg halves at the one end of the leg halves, d) assembling the yoke segment at the one end of two halves of the core leg, where the yoke segment is matching to the shape of the ends of the two opposite halves of the core leg, e) positioning an inner clamping beam on a surface of the yoke segment and the two halves of the core leg and tightening both clamping beams and together forming a first clamp of the frame, f) forming a second clamp of the frame at the other end of the two halves of the core leg by repeating the steps "c" to "e", g) tightening the first clamp and the second clamp together by securing means, h) rotating a single core frame into vertical position and releasing securing bar of the assembly stand, or releasing the securing bar of the assembly stand first, and next lifting a single core frame into vertical position, i) assembling two additional single core frames, by repeating steps "a"-"h", j) positioning the all three single core frames in abutting position, and then tightening them mechanically at the bottom and the top of the frames. a) positioning of two halves of one leg on the base plate of the assembly stand on the horizontal position using a lower part of adapters for maintain the proper angle between the two halves of the one leg, b) securing the positioned halves using an upper part of adapters and the securing bar placed on steel rods, c) positioning of an outer clamping beam underneath two leg halves at the one end of the leg halves, d) assembling the yoke segment at the one end of two halves of the core leg, where the yoke segment is matching to the shape of the ends of the two opposite halves of the core leg, e) positioning an inner clamping beam on a surface of the yoke segment and the two halves of the core leg and tightening both clamping beams and together forming a first clamp of the frame, f) forming a second clamp of the frame at the other end of the two halves of the core leg by repeating the steps "c" to "e", g) tightening the first clamp and the second clamp together by securing means, h) rotating a single core frame into vertical position and releasing securing bar of the assembly stand, or releasing the securing bar of the assembly stand first, and next lifting a single core frame into vertical position, i) assembling two additional single core frames, by repeating steps "a"-"h", j) positioning the all three single core frames in abutting position, and then tightening them mechanically at the bottom and the top of the frames.