An algorithm to recover an unterminated turbo-code interleaver is proposed. It is found that for some rational functions, their series are periodic, and then the sum of two bits from the info sequence and two bits from the corresponding interleaver sequence is 0. There comes out a recognition algorithm: two interleaved positions are recovered first; other positions would be recovered step by step. The algorithm is sliced into several lists, and the number equals to the period.