An active guarding technique is demonstrated for substrate immunity improvement on LC-tank oscillators. Active components in the oscillator, which are sensitive to substrate noise and contribute to the output most, are guarded from the substrate perturbation by applying the active guarding circuit to decouple and also compensate the noise current. To evaluate the effectiveness of the proposed technique, noise signals of both single and multiple tones of various frequencies are injected to the substrate close to the guarded transistors and measure the sideband spurs by switching the active guarding circuit on/off respectively. Experimental results showed more than 11 dB noise suppression performances can be achieved in a wide frequency range from DC to 10 GHz. By applying the active guarding circuit to a LC-tank oscillator, more than 17 dB spur suppression performance is obtained in the frequency range of interest.