In the last years, worldwide scientific communities working in the area of computational biology and medicine have shown an increasing interest to the problem of computation of electromagnetic fields in the human body and safety aspects of human exposure to RF radiation. In this paper we investigate the influence of the frequency of mobile phones upon the electromagnetic energy absorption in tissues and express it as specific absorption rate (SAR) calculated with an algorithm based on Finite Integration Technique for a mobile phone excited with two different frequencies: 900 MHz and 1800 MHz. A three-dimensional human model based on anatomical data, which describes the conductivity distribution in the human body, is used in these simulations.