A novel dual-band out-of-phase power divider (PD) with impedance transformation and wide frequency ratio is proposed in this letter. This proposed circuit consists of four double-sided parallel-strip lines (DSPSLs) to obtain the out-of-phase and dual-band performance, mid-inserted conductor plane as a common ground, and two resistors to improve the output port isolation. A through ground via (TGV) is employed to realize the high power application. By using rigorous even- and odd-mode analysis, a set of closed-form equations and design procedures are obtained and the analysis indicates that this PD can operate at two arbitrary frequencies with wide frequency ratio $(2.44leq k leq 7.31)$ . Furthermore, an experimental circuit is designed, and fabricated. The simulated and measured results agree very well to verify our design theory.