We consider the Sobolev type spaces H σ (K) with σ ≥ 0, where K is a postcritically finite self-similar set with the natural boundary. Firstly, we compare different classes of Sobolev spaces H σ N (K), H σ D (K) and H σ (K), and observe a sequence of critical orders of σ in our comparison theorem. Secondly, We present a general atomic decomposition theorem of Sobolev spaces H σ (K), where the same critical orders play an important role. At the same time, we provide purely analytic approaches for various Besov type characterizations of Sobolev spaces H σ (K). 22010 Mathematics Subject Classification. Primary 28A80.