This work describes the mechanical and the electromagnetic design of a microwave surface applicator used to coagulate liver tissue in the treatment of hepatic tumors. A good prediction of the ratio between reflected and forward microwave power (return loss) is obtained with a finite element model using commercial software. Laboratory testing of the applicator performed in polyacrylamide gel (PAG) and in ex vivo bovine liver show a hemispherical heat distribution pattern and hemispherical ablations up to 20 mm in diameter and 15 mm in depth in a controlled manner in 1 min. The applicator can also be used to coagulate larger areas of tissue with 2–5 mm depth by moving the applicator on the surface of the tissue. Experimental results indicate that the coagulated volume of tissue is approximately proportional to the energy delivered into ex vivo bovine liver, hemispherical in shape, obtained in short time duration with a volumetric rate of coagulated tissue of about 50 mm3 /s.