This letter proposes a type of slow-wave broadside-coupled microstrip lines. The new structure only requires a single-layer substrate and has the independently controllable even- and odd-mode impedances and slow-wave factors by adjusting the corresponding structural parameters. This property greatly simplifies the circuit design. In addition, it provides a large coupling strength. A miniaturized wideband microstrip rat-race coupler with the high isolation and perfect balances is implemented using the proposed slow-wave coupled lines.