Recently, the orthogonal frequency-division multiple access (OFDMA) based wireless communication system is usually used in interactive digital video broadcasting environments or the wireless metropolitan area network (WMAN). Therefore, the relative condition of the inter-user channel can be easily deteriorated. This letter proposes and evaluates the mutual amplify-and-forward (AF) cooperation between two users using modified space time block code (STBC) to provide the reliability for an OFDMA system with a single antenna in an inferior inter-user channel environment. As a result, an OFDMA system adopting the proposed cooperation becomes more robust to inferior inter-user channels than our previously proposed decode-and-forward (DF) cooperation.