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Homogeneous illumination is important in microwave-induced thermoacoustic tomography (TAT) because it directly determines the quantification of the reconstructed images of objects. In this work, we studied the effects of both linearly and circularly polarized microwave illuminations on TAT images. A customized horn antenna and a helical antenna working at 3 GHz were used to provide linearly polarized and circularly polarized illuminations, respectively. Based on our simulated and experimental results from tumor phantoms, the imaged patterns of the targets showed dependence on their permittivity. The target size and microwave wavelength within the target affected the microwave power distribution, which resulted in different reconstructed images. We analyzed the effect of the microwave penetration depth on TAT images as well. The images under circularly polarized illumination presented better quality than …