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In order to assess the compliance of ingested wireless device (IWD) with related international safety guidelines, the studies on the biological effects and the signal intensity of an IWD in two realistic human body models using the finite-difference time-domain (FDTD) method are reported in this paper. Simulation studies are carried out in 21 scenarios where the IWD is placed at seven positions with three orientations, for each of the two human models. Specific absorption rate (SAR), temperature rise, near fields and far fields are analyzed in the 21 scenarios at four selected operation frequencies of 430 and 800 MHz, and 1.2 and 2.4 GHz, respectively. Our study indicates that the radiation intensity outside of the human body decreases with the increase of operation frequency. Furthermore, the radiation characteristics of the IWD are orientation and position dependent. The polarization of electric field outside of the human body is similar to that of the radio frequency (RF) source. As far as the compliance of safety is concerned, among all the simulated scenarios the maxima of the averaged-1 g and averaged-10 g SARs can reach 3.71 W/kg and 1.37 W/kg at the input power of 25 mW, respectively.