||Direct-conversion receivers have become popular in radio-frequency (RF) circuit design for their advantages of low cost, low power consumption, and fewer chip components over other architectures such as heterodyne receivers. However, the direct-conversion receiver architecture often suffers from direct current (dc) offset, which is a consequence of the imperfect direct-conversion process. In this paper, we study the effects of dc offset on the symbol error rate (SER) performance of orthogonal frequency-division multiplexing (OFDM) systems in multipath Rayleigh fading channels. Since OFDM system performance is sensitive to carrier frequency offset (CFO), the CFO must be estimated and compensated at the receiver. Due to CFO compensation, the dc offset caused by direct-conversion receivers and/or mixed-signal circuits no longer only affects the dc subcarrier and is spread over all subcarriers. By deriving the analytical SER formulas for OFDM systems with various modulation formats, the dependency of SER on dc offset and CFO is clearly quantified. These SER formulas can help system designers determine suitable specifications of RF components and understand whether digital DC offset compensation is necessary or not. Finally, we propose and analyze a simple DC offset estimation and cancellation scheme under the assumption that DC offset holds constant in one OFDM symbol duration. Numerical results demonstrate the effectiveness of the proposed DC offset-cancellation scheme.