Multi-phase RF ablation employing a two-dimensional or three-dimensional electrode array produces a multitude of currents paths on the surface of the ablation zone. This results in a uniform lesion with a size defined by the span of the electrode array. An orthogonal electrode catheter array suitable for cardiac ablation is used in conjunction with a two-phase RF power source to produce uniform square-shaped lesions of size 1.2 cm.sup.2. Lesions of larger size are created by successive adjacent placement of the square-shaped lesions. A temperature sensor at the electrode tip allows monitoring of ablation temperature and regulation of thereof to minimize the electrode tips from being fouled by coagulum.