In this brief, the design methodology of microstrip impedance transformers with extreme (i.e., extremely high and extremely low) impedance transforming ratios is proposed by deploying the cross-shaped structure (CSS) configuration. The proposed impedance transformer can significantly eliminate the line width limitation of traditional quarter-wave impedance transformer and hence an extremely wide range of impedance transformation can be easily realized. At the same time, the impedance transformer provides a good frequency selectivity, especially in the narrowband regime. To demonstrate the outstanding performance of proposed CSS impedance transformer, two examples, with the load-to-source impedances of 2000-to-50 Ω and 5-to-50Ω, respectively, are presented and prototyped. The measurement results are in good agreement with the simulated counterparts and hence, the outstanding performance of extreme impedance transforming ratios is validated.