Abstract: The present invention is related to an improved transformer structure including a main body and, accommodated in the main body, a first winding frame and two winding frames. Hollow portions of the first winding frame and each of the second winding frames are combined with openings on the main body, for forming a primary side passage in a longitudinal direction and two secondary side passages in a lateral direction, so as to penetrate respectively a first magnetic material into the primary side passage and a second magnetic material into the two secondary side passages. Through the longitudinal primary side passage and the lateral secondary side passages, the primary and the secondary side winding can have a proper interval therebetween for achieving a stable output, so that a high voltage can be directly applied to the transformer for driving the large size LCD back light module for improving the efficiency.