A catheter comprises a catheter body with an ablation assembly at its distal end. The ablation assembly has a preformed generally circular curve generally transverse to the axis of the catheter body, and comprises a flexible tubing that carries a tip electrode at its distal end. In use, the generally circular curve contacts the inner circumference of a tubular region of a patient's heart so that the tip electrode contacts tissue along the inner circumference. After ablating the tissue at this first position, the ablation assembly can be rotated so that the tip electrode contacts tissue at a second position along the inner circumference. After ablating the tissue at the second position, the procedure can be repeated to form a lesion of the desired length.