In an RFID system, it is desirable to have both the reader and the transponder tuned to the same resonant frequency for efficient data transmission between them. Any difference in frequency will decrease the transponder coil voltage or the internal power supply voltage and will increase the possibility of zero modulation in the reader coil, which results in the reduction of the reading distance. In this paper, the phenomena caused by the frequency mismatch are theoretically analyzed and mathematically modelled. Several schemes to compensate for the frequency mismatch are also mentioned. The derived equations and analyzed theory on the data transmission between the reader and the transponder will be helpful to the development of RFID systems for many applications.