A new theory of designing dual-band power divider (PD) based on Wilkinson structure is presented in this paper. Because of the utilization of coupled lines, the proposed PD is with a high design freedom that many transmission line (TL) combinations can be used for a fixed frequency ratio k (ƒ2/ƒ1). Meanwhile, since the lengths of the conventional coupled lines used are extremely long, the slow-wave structure is introduced, which configuration results in a property of designable length and compact size because of the improved space efficiency. After analysing the even- and odd-mode circuits, precise design formulas are derived and further verified for the frequency ratio from 1.5 to 6.0 using NI AWR simulator under the consideration of the largest realizable TL impedance as 128 Ω. For verification, two PDs with the same centre frequency of 1.0 GHz and 4.0 GHz but different TL configurations are designed based on the proposed design theory. According to the experimental results, both fabricated PDs have good performances as expected, such as low in-band insertion loss (S21), wide isolation, and relatively better out-of-band rejection than majority of previous studies that the maximum S21 between the two passbands is larger than 6 dB, while previous studies are around 4 dB.