Dual-Function Radar and Communication (DFRC) systems have emerged to solve spectrum scarcity by integrating both Radar and communication functionalities based on a single waveform. Recently, a great attention has been paid to the systems based on multiple antennas and using orthogonal frequency division multiplexing (OFDM) as a waveform. In that case, one of the key issues is the estimation of the target parameters such as the direction of arrival (DOA), the range and the velocity. In this paper, we derive what is called the modified Cramér-Rao bound (MCRB) for the estimations of the DOA, range and velocity of the targets and analyze how the MCRB depends on the signal-to-noise ratio and the system parameters such as the number of sub-carriers, OFDM symbols and antennas.