The principle of a TWT oscillator using a YIG as feedback element is analyzed and the tunning performance is derived also. It shows also that this kind of oscillator can oscillate in a rather wide frequency range but the oscillating frequency is separated in bands. There are forbidden regions for oscillating. Under ordinary conditions, the forbidden regions are almost equal to the oscillating regions. The results of experiments are in agreement with the theory. This shows that there still are problems if one uses the TWT-YIG oscillator as a broad-band sweeping oscillator.