Intermittency are commonly observed by practicing power supply engineers in their design workbenches. It shows to be a symmetrical period-doubling bifurcation in time domain with fixed long intermittent period. Sometimes it is called "breathing". Such kind of operation mode is usually undesired and can be avoided by means of screen technique or modifying the circuit parameters. This paper explore the mechanism and condition for the emergence of intermittency in a voltage-mode controlled Buck converter, and thus conclude that intermittency is a common and complex phenomenon by the fact that any interference, with frequency approaching switching frequency, or switching frequency's multiples or fractions, will induce the intermittency. Furthermore, the type and period of intermittency, and also the initial operation state have relations with the interference strength and frequency separately.