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A modified set partitioning in hierarchical trees (SPIHT) algorithm based on wavelet coefficient blocks, which is more appropriate to robust image transmission over noisy channels when using unequal error protection (UEP) scheme, is presented. According to the algorithm characteristic of traditional SPIHT coding that scanning once can process four wavelet coefficients, our approach divides the wavelet transform coefficient matrix into corresponding coefficient blocks and encodes every block with different strategies. When the maximum value of the coefficient block is less than the current threshold, only one bit is used for coding, thus to reduce the loss of coding efficiency caused by repeat coding and early coding for non-important coefficients. Simulation results show that the proposed algorithm combined with UEP can provide a better peak signal to noise ratio (PSNR) performance for image transmission over noisy channel, especially at low bit rate.