A turbo decoding method and apparatus for performing iterative decoding on a received signal is provided. A decoder receives a signal and an (N−1)th decoding result, performs Nth decoding thereon, and outputs first extrinsic information. A transformer reduces a bit width of the first extrinsic information output from the decoder and outputs second extrinsic information. A memory stores the second extrinsic information output from the transformer. A de-transformer de-transforms the second extrinsic information stored in the memory, to third extrinsic information having a same bit width as that of the first extrinsic information, and inputs the third extrinsic information to the decoder.