Single-crystalline spherical nearly monodisperse tellurium (Te) nanocrystals (NCs) withaverage diameters of 20 and 90 nm, respectively, have been fabricated for the first time by afacile solution sonochemistry process. The structural characterizations show thatthe as-synthesized Te NCs have pure hexagonal structure, as revealed by x-raydiffraction (XRD), selected-area electron diffraction (SAED), energy-dispersivex-ray (EDX) spectroscopy, and high-resolution transmission electron microscopy(HRTEM) methods. The size-dependent structural phase transition of Te NCs up tothe high pressure of 20 GPa has been investigated in a diamond anvil cell usingresistance measurement at room temperature, and compared with the behaviorof bulk Te under identical conditions. The experimental results indicate that20 nm Te NCs, 90 nm NCs, and bulk Te all undergo two phase transitions upto 20 GPa, their respective transition pressures being about 7.2 and 10.3 GPa,5.9 and 8.8 GPa, and 4.0 and 6.8 GPa. This indicates that the phase transitionpressures are higher for the smaller NCs. In this paper we discuss the size-dependentstructural phase transitions, the sluggishness of the phase transition process,and the fluctuating properties of the phase transition products at high pressure.The present work might open an avenue to real-time detection of the dynamicsof the phase transition in bulk and nanoscale materials at high pressure, andalso could serve as a guide to tailoring the microscopic properties of materials.