In this paper, the low-frequency oscillation phenomenon in a pulse train (PT) controlled buck converter operating in continuous conduction mode (CCM) is found and studied. The mechanism of this low-frequency oscillation phenomenon and the way to suppress it are studied. The time-domain simulation results and their corresponding phase portraits indicate that the equivalent series resistance (ESR) of the output filter capacitor has a critical effect on this low-frequency oscillation phenomenon. When the ESR is zero or small enough, such a low-frequency oscillation phenomenon appears in the PT controlled CCM buck converter because the output voltage can not be regulated timely. With the increase of ESR, the PT controlled CCM buck converter operates chaotically. The output voltage can be regulated timely and such a low-frequency oscillation phenomenon can be suppressed effectively. Experimental results are provided to verify the theoretical analyses and the simulations.