The present application proposes a transient enhancing circuit for a constant-on-time converter. The constant-on-time converter includes an error amplifier and a comparator. The transient enhancing circuit includes a first sample-and-hold circuit and a zero-current detection circuit. The first sample-and-hold circuit has an input terminal and an output terminal. The input terminal of the first sample-and-hold circuit is coupled to an output terminal of the error amplifier, and the output terminal of the first sample-and-hold circuit is coupled to a first input terminal of the comparator. The zero-current detection circuit is coupled to the first sample-and-hold circuit and arranged for outputting a control signal when current flowing through a load of the constant-on-time converter is detected to be zero. The present application also proposes a constant-on-time converter using the transient enhancing circuit.