Abstract: Let T_X be the full transformation semigroup on a nonempty set X and E be an equivalence on X. Then T_(X)=f∈T_X:x,y∈X,(f(x),f(y))∈E(x,y)∈E is a subsemigroup of TX of transformations reflecting the equivalence E. Fix an element θ∈T_(X) and define an operation ° on T_(X) by f°g=fθg where fθg denotes the composition of the maps g,θ and f in the usual sense. With respect to the new operation °, T_(X) forms a new semigroup which is called a sandwich semigroup of T_(X) and denoted by T_(X;θ).  The regular elements of the sandwich transformation semigroup T_(X;θ) were characterized and a necessary and sufficient condition for the regular semigroup T_(X;θ) was presented.