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Flattening is a method to make a definite clause function-free. For a definite clause C, flattening replaces every occurrence of a term f(t1, . . . , tn) in C with a new variable v and adds an atom pf(t1, ...,tn,v) with the associated predicate symbol pf with f to the body of C. Here, we denote the resulting function-free definite clause from C by flat(C). In this paper, we discuss the relationship between flattening and implication. For a definite program Π and a definite clause D, it is known that if flat(Π) = flat(D) then Π = D, where flat(Π) is the set of flat(C) for each C Ɛ Π. First, we show that the converse of this statement does not hold even if Π = {C}, that is, there exist definite clauses C and D such that C = D but flat(C) ≠ flat(D). Furthermore, we investigate the conditions of C and D satisfying that C |= D if and only if flat(C) |= flat(D). Then, we show that, if (1)C is not self-resolving and D is not tautological, (2) D is not ambivalent, or (3) C is singly recursive, then the statement holds.