Abstract: This paper describes an automatic scaling method for extracting the trace and parameters from oblique ionogram. The proposed technique adopts quasi-parabolic(QP) model to describe the electronic concentration distribution of F₂ layer. The secant theorem, Martyn's equivalent path theorem, image processing technology and echo characteristics of ionogram are used to determine the final values of some parameters. Then, the hybrid genetic algorithm is used to find a set of optimal parameters within the scope of search spaces to make sure that the synthesized F₂ layer trace accords with the real trace best. In order to verify the performance of this method, 120 oblique ionograms are scaled and their results are compared with manual scaling results and vertical data. Comparison results show that the method can accurately scale oblique ionogram parameters.