We obtain a characterization of modulus of smoothnes of fractional order in the Lebesgue spaces L p ω , 1 < p < ∞, with weights ω satisfying the Muckenhoupt's A p condition. Also, a realization result and equivalence between modulus of smoothness and the Peetre K-functional are proved in L p ω for 1 < p < ∞ and ω ∈ A p. Theorem 1 ([31]). Suppose that r > 0 and 1 ≤ p ≤ ∞. In this case (a) If f ∈ L p , then there exists a function ϕ ∈ Φ r such that ϕ (t) ≈ ω r (f, t) p holds for any t ∈ (0, ∞), where equivalence constants are depend only on r and p. (b) If ϕ ∈ Φ r , then there exist f ∈ L p and a positive real number t 0 such that ω r (f, δ) p ≈ ϕ (δ) holds for any δ ∈ (0, t 0 ], where the equivalence constants are depend only on r and p.