The most exciting and potentially important development in coding theory in recent years has been the dramatic announcement of “Turbo codes". Turbo codes are constructed by applying two or more component codes to different interleaved versions of the same information sequence. Then, the encoded bits are decoded through an iterative decoding algorithm of relatively low complexity. Turbo code decoder consists of two soft-input/soft-output component convolutional decoders that work together in an iterative fashion. Turbo code decoding process is the most complex part of the turbo coding decoding process. Turbo decoding process has a lot of ambiguity and scarce of decoding details in the literature publication. Due to of these, the goal of this paper is shading the light to the mechanism of the turbo decoder to illuminate the principles of the iterative decoding process of the turbo code decoder through a numerical example applied to the actual turbo decoder.