Provided are a method of estimating an encoder for dual stream transmission, and an encoder using the same. The method includes the steps of: a) setting up the number of memories required for an encoder, a free distance condition for robust data, and a free distance condition for mixed stream of robust data and normal data; b) obtaining ½ rate encoder combinations that sustain backward compatibility with a typical trellis encoder; c) obtaining encoder combinations H that satisfy the free distance condition for the robust data and the free distance condition for the mixed stream from the ½ rate encoder combinations; d) obtaining relations between memories and input/output signals, which satisfy the encoder combinations H; and e) configuring an encoder according to the obtained relation between the memories and the input/output signals.