Transition metal oxides semiconductors (TMOs) are known for their special optical and electrical properties with wide-ranging applications, including gas sensing, storage devices such as Li-ion batteries, solar cells, and catalysts[1,2]. Among different types of TMOs, there is a class of materials that are distinguished by their unique layered structure and multiple oxidation states, such as MoO3, WO3, and V2O5. The molybdenum trioxide (MoO3) is known for its photo-, thermo- and electrochromism, high catalytic activity[3]. The MoO3 has found in different structural phases including the orthorhombic phase, ??-MoO3; monoclinic phase, ??-MoO3; metastable phase at high-pressure conditions, ?????-MoO3; and hexagonal phase, h-MoO3[4]. Among them, the ??-MoO3 is the most stable crystal phase and it has a layered structure consisting of van der Waals bonded sheets of distorted edge-sharing Mo???O6 octahedra in which Mo atoms are bounded by three distinct types of oxygen atoms[5]. In this research, the structural properties and hyperfine parameters of 111mCd(111Cd) impurities in ??-MoO3 are investigated by first-principle calculations in the framework of density functional theory (DFT). The Perdew???Burke???Ernzerh of generalized gradient approximation (GGA-PBE), and GGA-PBE plus Hubbard-U corrections for onsite Coulomb interactions are used in the DFT calculations. In the calculations performed, the effect of van der Waals forces between layers is employed using the DFT-D3 method[6]. To interpret the experimental results, different configurations around the Cd atom including the different types of oxygen vacancies are simulated. The comparison of experimental data with calculated hyperfine parameters indicates that the Cd atom is predominantly located in the interstitial lattice site of MoO3, and also the oxygen vacancy is most likely to form on the 2-fold coordinated (O2) atoms. The results of this work demonstrate the benefit of first-principle calculations for solving the outstanding questions arising from the experiment.