In this paper, we present an analytical approach to design the inner region size for the fractional frequency reused (FFR) time division duplex (TDD) orthogonal frequency division multiple access (OFDMA) systems when supporting asymmetrical traffic services. In TDD system, supporting asymmetrical traffic can be achieved by adjusting the ratio of downlink and uplink transmission period. Nevertheless, the intercell interference may be deteriorated by the cross-slot interference resulting from the different downlink and uplink ratios among neighboring cells. Although the FFR technique can overcome the inter-cell interference, the impact of the cross-slot interference of TDD systems due to various downlink to uplink traffic ratios in different cells on the inner region size of a FFR-based OFDMA system is an open issue to our best knowledge. Intuitively, the larger the inner region of a FFR system, the higher the system capacity. Thus, this paper can provide a systematic method to determine the size of the inner region of FFR-based OFDMA system for different ratios of the downlink to uplink traffic among two neighbor cells.