This paper presents the development of high power-density vibration micro energy harvesters (VEHs) exploiting MgHf-codoped AlN ((MgHf)xAl1-xN) piezoelectric films. The devices provided a significant improvement in performance of micro-energy harvesters. A normalized power density (NPD) of 34.9 mW.g-2.cm-3 was increased 18-fold than that of the best published AlN-based device. The devices in this study consisted of a (MgHf)xAl1-xN/Si cantilever and a W/Si proof-mass. Our piezoelectric films with MgHf dopant of χ = 0, 0.05, and 0.10 were grown successfully on Pt/Ti/SOI substrates by an ion-beam sputtering using an AlN and a MgHf targets in Ar/N2 ambient. The films offered a power-generation figure of merit (FoM) of 22 GPa, which is 3-fold larger than that of AlN. The results confirmed the high potential of (MgHf)xAl1-xN-based VEHs as power sources for wireless sensor networks (WSNs) crucial to realize Internet of Things (IoT).