Background: Anterior line ablation for peri-mitral atrial flutter (AFL) is associated with biatrial flutter due to disruption of the electrical conduction in the left atrial septum. Tachycardia cycle length (TCL) prolongation during anterior line ablation of peri-mitral AFL can be due to change of the AFL into a single loop biatrial flutter. Method: An AFL case with valvular disease, cardiac surgery and prior ablation was confirmed to be counterclockwise peri-mitral flutter with isthmus identified on the left atrial septum by LAT, bipolar voltage and propagation mapping. Ablation on the anterior and septum left atrium (LA) targeting the isthmus prolonged TCL from 266 ms to 286 ms. A second map was performed for both the left atrium (LA) and the right atrium (RA). Result: The activation in LA remained to be counterclockwise, but there was interruption of the LAT. The reentrant circuit was counterclockwise single loop biatrial, involving the whole LA and the RA septum with Bachmann bundle and the inferior posterior septum being the connection between RA and LA. The AFL was terminated by alation at the RA superior septum below SVC. Conclusion: Counterclockwise peri-mitral flutter can change into single loop biatrial flutter during anterior line ablation. RA mapping should be considered if there is prolonging of TCL without termination of the AFL and if there is interruption of LAT. The biatrial flutter can be terminated by ablation at the Bachmann bundle insertion in the RA.