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In the sub-channel reliability based HARQ (sRB-HARQ) scheme the channel qualities are exploited in retransmission to enhance the system throughput. The sub-channels with poor channel qualities measured by signal to noise ratio (SNR) are considered to be more unreliable, and the data on such sub-carriers are prior to be retransmitted when necessary. In this paper, an improved sRB-HARQ scheme for OFDM system is proposed, and the retransmission amount of sub-channels is determined adaptively by the target reliability of the received information which is related with the error ratio of received data (e. g. BLER in this paper). The target reliability for each (re)transmission could be same or different which are categorized as single target and multi target respectively. The sub-channels with least reliabilities are retransmitted on better sub-channels in order to achieve higher diversity gain. Thus the system throughput could be improved significantly and high power efficiency is achieved which are also proved by simulation results.