In this study, the finite-difference time-domain (FDTD) method is used to model mutual effects of a mobile phone and a human head in terms of both biological effects and antenna design. A discrete human head model and a hand-held receiver with various antennas mounted on top of ii, are located within a three dimensional (3D) FDTD algorithm is built in cartesian coordinates. Near fields are simulated directly in time domain for both sinuzoidal and pulse type antenna excitations. The antenna power, input impedance, absorbed power and the specific absorbtion rate (SAR) distribution inside the head near the hand-held receiver are calculated for various antenna types and human head-hand-held receiver locations. The simulations are carried out for both European GSM and DECT systems (at 900 and 1800 MHz, respectively). The SAR distributions for various vertical and horizontal slices of the head are calculated and are shown to agree with the available calculation and measurement results. Besides, the effect of human head to the antenna radiation pattern are calculated where far field simulations are obtained via a time-domain near-to-far-field (NTFF) transformation based on the Huygen's principle. Various hand-held receiver antennas-including quarter-wavelength wire, IFA, PIFA, symmetrical and asymmetrical square or rectangular printed loops-are investigated in terms of their radiation pattern, gain and efficiency. Different antenna mountings are used and their effects to both antenna performance and SAR distribution in the head are shown.