A turbo encoded H-ARQ system and an error detection method are disclosed, and more particularly, a turbo encoded H-ARQ system and an error detection method that lower an undetected error probability and frame error rates by increasing the degree of an equivalent CRC generator polynomial by using a modified trellis termination. It has been made possible to obtain an equivalent CRC generator polynomial having higher degree than the related art CRC generator polynomial by employing a modified trellis termination. This achieved a lower undetected error probability than the related art undetected error probability by using an equivalent CRC generator polynomial having higher degree. This also achieved lower frame error rates, when the same number of overhead bits are used, due to the lower undetected error probability.