Given β ∈ (1, 2) the fat Sierpinski gasket S β is the self-similar set in R 2 generated by the iterated function system (IFS) f β,d (x) = x+d β , d ∈ A := {(0, 0), (1, 0), (0, 1)}. Then for each point P ∈ S β there exists a sequenceand the in nite sequence (d i ) is called a coding of P. In general, a point in S β may have multiple codings since the overlap region O β := c,d∈A,c =d f β,c (∆ β ) ∩ f β,d (∆ β ) has non-empty interior, where ∆ β is the convex hull of S β . In this paper we are interested in the invariant set U β := ∞ i=1 d i β i ∈ S β : ∞ i=1 d n+i β i / ∈ O β ∀n 0 . Then each point in U β has a unique coding. We show that there is a transcendental number β c ≈ 1.552 63 related to the Thue-Morse sequence, such that U β has positive Hausdorff dimension if and only if β > β c . Furthermore, for β = β c the set U β is uncountable but has zero Hausdorff dimension, and for β < β c the set U β is at most countable. Consequently, we also answer a conjecture of Sidorov (2007).Our strategy is using combinatorics on words based on the lexicographical characterization of U β .