Recently, with the urgent need for renewable and clean energy sources, the emphasis has been on extracting and generating energy from the surrounding environment. Harvesting mechanical kinetic energy is an attractive field for easy access to energy from multiple sources, and transforming it out of the mechanical domain to the electrical domain. Resonant systems introduced a magic solution in maximizing the output power when the ambient frequency is similar to the natural frequency of the mechanism. But the main drawback of these systems remains when moving away from the resonance zone even slightly, where that leads to a significant reduction in the output power. This paper discusses the design of an innovative autonomous mechanism for tuning the resonant frequency and thereby generate larger amounts of energy. Unlike previous researches which rely heavily on tuning the resonant frequency by changing the spring stiffness, the proposed concept is based on adjusting the resonant frequency by automatically modifying the mass moment of inertia. The proposed energy harvester used the electromagnetic technique to transform mechanical vibration into electric power. This design is analyzed using the methodology of a vibration continuous system. Different parameters of the tuning mechanism have been investigated to achieve the resonant. The tuned mechanism has been examined experimentally and then validated by comparing experimental results with the results obtained through the analytical model.