We present the electromechanical and microwave properties of a novel micromachined parallel-plate tunable capacitor with a wide tuning range. Different from conventional two-parallel-plate tunable capacitors, this novel tunable capacitor consists of one suspended top plate and two fixed bottom plates. One of the two fixed plates and the top plate form a variable capacitor, whereas the other fixed plate and the top plate are used to provide electrostatic actuation for capacitance tuning. For the fabricated prototype tunable capacitors, a maximum tuning range of 69.8% has been achieved experimentally, exceeding the theoretical tuning range limit (50%) of conventional two-parallel-plate tunable capacitors. This novel tunable capacitor also exhibits very low return loss (< 0.6dB between 45 MHz and 10GHz).