An ultra-wideband Voltage-Controlled Oscillator based on the Colpitts structure is proposed in this work. The entire circuit consists of four Colpitts VCO cores operating at different frequency. Every VCO core employs two single-ended Colpitts oscillators arranged back-to-back and sharing a common LC resonating tank. The coupling coefficient between the two single-ended Colpitts oscillators is increased by introducing a transformer placed on the emitter node of the core transistor, which ensures two Colpitts oscillators oscillating in a differential mode. Furthermore, a series of amplifier circuits, including a fundamental frequency amplifier, second harmonic amplifier, and frequency multiplier, are employed to amplify the second harmonic signal and convert it into a differential signal, thereby expanding the operating bandwidth of the oscillator. The fundamental frequency of the proposed Colpitts VCO ranges from 3.5 GHz to 7GHz, and the frequency can reach up to 14GHz or 28GHz by generating second or fourth harmonics. The measured phase noise performance is lower than -130dBc/Hz, -124dBc/Hz and -118dBc/Hz at 1MHz offset across the entire frequency range. The power dissipation of the entire VCO is typically around 60mW under 3.3V power supply.