In this paper, a compact and flexible spiral antenna is designed for superficial tumor treatment having different shapes and sizes. The proposed antenna consists of a spiral radiating patch with a coplanar waveguide feed. The antenna is designed on FR4 substrate having a thickness is 0.15 mm and a size of the antenna is 18 × 19 mm 2 . The performance of a flexible spiral antenna (FSA) is tested in the presence of a heterogeneous phantom (skin, fat, and muscle) and also with a replicate of the human model (Voxel model). The bending effect of FSA is studied with a voxel model. The performance of FSA is evaluated in terms of specification absorption rate (SAR), and relative parameters of SAR i.e. penetration depth (PD) and effective field size (EFS). The performance of FSA is also analyzed for inserting different shapes and sizes of superficial tumors in the phantom and also studied the steady state and transient state temperature distribution in the phantom and gustav model.