Based on the standardized IEEE 802.11 Distributed Coordination Function (DCF) protocol, this paper proposes a new backoff algorithm, called Smart Exponential-Threshold-Linear (SETL) Backoff Algorithm to enhance the system performance of contention-based wireless networks. As we know, the smaller contention window (CW) will increase the collision probability, but the larger CW will delay the transmission. Hence, in the SETL scheme, a threshold is set to determine the network load. When the CW is smaller than the threshold, a light network load, the CW size is self-adjusted exponentially. Conversely, if the CW is larger than the threshold, a heavy network load, the CW size is tuned linearly. In addition, the SETL takes a more conservative measure by decrease the CW after "S" times consecutive successful transmission to reduce the collision probability, especially when the competing station is large. By simulation, the numerical results show that the SETL provides a better system throughput and collision rate in both light and heavy network load than the related backoff algorithm schemes, including binary exponential backoff (BEB), exponential increase exponential decrease (EIED) and linear increase linear decrease (LILD). The SETL is very easy to implement, as it dose not require any changes in DCF procedures. Every station will self-adjust CW well with high performance and low collision rate.