The present disclosure relates to a ring-oscillator with glitch-free frequency-tuning. The disclosed ring-oscillator at least includes multiple delay stages coupled in series within a ring loop and having a first delay stage, a capacitor bank coupled between an output of the first delay stage and ground, and a timing block configured to receive an output signal of the first delay stage and at least one controlling signal. The at least one controlling signal determines at least one capacitor in the capacitor bank connecting or disconnecting to the ring loop. The timing block is configured to pass or not pass the at least one controlling signal to the capacitor bank based on whether the output signal of the first delay stage meets a certain condition. Therefore, the connection or disconnection of the at least one capacitor does not cause a significant voltage change at the output of the first delay stage.