Behavior of the muonium in two polycrystalline CVD diamond samples and in the sample composed of few single-crystal synthetic diamonds was studied by transverse-field mu SR measurement. The hyperfine interaction constants for muonium in the synthetic samples are in agreement with those in natural diamond. The relaxation rate of muon spin and the amount of the Mu(T) fraction in the synthetic samples are close to those observed in IIa- and IIb-type single-crystal natural diamonds. (C) 2012 Elsevier B.V. All rights reserved.