A one-coil multi-core inductor-capacitor (LC) oscillator is provided. The one-coil multi-core LC oscillator includes a main coil and at least one mode suppression device. The main coil includes an outer wire and a central wire, wherein the outer wire is coupled to a first core circuit and a second core circuit, and the central wire is coupled between a first node and a second node of the outer wire. More particularly, an outer loop formed by the outer wire corresponds to a first mode of the one-coil multi-core LC oscillator, and inner loops formed by the outer wire and the central wire correspond to a second mode of the one-coil multi-core LC oscillator, where the at least one mode suppression device is configured to suppress one of the first mode and the second mode.