Mechanical energy harvesting has received substantial attention as a sustainable power generation technology for self-powered wireless sensor network systems in biomedical, wearable as well as industrial health monitoring applications. There are abundant mechanical sources available in nature including sound, vibration, ultrasonic waves and human-based kinetic energies. These sources can be converted into useful electrical energy via piezoelectric materials and devices that are advantageous due to its high efficiency and direct conversion mechanism. Although piezoelectric energy harvesting is a very attractive technology, insufficient power generation still remains as an issue to overcome in order to realize a self-powered system for practical use. So far, development of high efficient piezoelectric materials, devices, and electrical circuits have been the key research approaches in order to enhance harvesting performance. In this work, we will present a new paradigm work, that is, enhancement of metamaterial-based energy harvesting. Metamaterials, artificially engineered materials, exhibit exotic properties including negative refractive index and bandgap, which thus enable us to manipulate mechanical wave propagations. In order to amplify input mechanical wave energy into energy harvesting systems, metamaterials can be utilized to guide and focus acoustic, elastic, vibration energies towards the desired position for harvesting, Recently, several research efforts on metamaterial-based enhancement of energy harvesting have been reported, but mostly based on intuitive design or with little experimental support. We propose an optimized design of phononic crystals with defect as metamaterial for piezoelectric energy harvesting system. Systematic design through geometric and bandgap optimization process is performed and followed by theoretical analysis and experimental verification. Drastic enhancement of energy harvesting performance via metamaterials, more than 20 times of power enhancement, is demonstrated and thoroughly investigated both analytically and experimentally.