Young's modulus determination of unpolled electrospun lead zirconate titanate (PZT) nanofibers under harmonic vibration using in situ scanning electron microscopy (SEM) is demonstrated. The PZT nanofiber was fabricated using an electrospinning process and collected on a silicon substrate with thermal oxide. After the annealing process, the random polarized PZT nanofibers were break and picked up for the Young's modulus measurement using tungsten tips of the nanomanipulator. By applying an oscillating electric field, PZT nanofibers were excited and the resonant frequency was observed through the live SEM image along with the transfer frequency spectra of a network analyzer simultaneously. The measured Young's modulus of random polarized PZT nanofibers varied from 46.6 GPa to 88.7 GPa calculated from the resonant frequency by Euler-Bernoulli method. This experimental result suggests that the unpolled electrospun PZT nanofibers have a similar Young's modulus with the PZT thin films.