A lightweight beam structure includes a first and a second frame. The first frame includes an upper plate, two inner side plates downward bent and extended from two sides of the upper plate, two outer side plates upward bent and extended from two inner side plates respectively and two fixing plates bent and extended from two outer side plates respectively. Each outer side plate is formed outside each inner side plate. A groove is formed between each inner side plate and each outer side plate. The second frame is connected with the first frame and includes a top plate and two intermediate plates downward bent and extended from two sides of the top plate. Each intermediate plate is received in each groove. The top plate is formed over the upper plate. Therefore, the stability is maintained and the weight is reduced so that the material costs may be decreased.