In this study, an experimental approach is used to characterize the geometrical and micromechanical properties of the shear transformation zones (STZs) in glassy polymers. Nanoindentation experiments have been conducted on both as-cast and annealed poly(methyl methacrylate) (PMMA) at different strain rates and temperatures, utilizing continuous stiffness measurement (CSM) technique at room temperature indentations, and conventional loading rate control method for nanoindentations at elevated temperatures. Employing a homogeneous flow theory for analyzing the experimental data, the geometrical properties of the STZs are observed to be almost independent of the thermal history of the samples. While the transformation shear strain of the STZ in PMMA is found to be slightly smaller than that in glassy metals, the size of the STZ in this polymer is shown to be at least 10 times of that in metallic glasses. On the other hand, the activation energy of a single STZ is found to change drastically with annealing. In addition, analysis of the rate sensitivity of the shear flow stress reveals a remarkable transition at a certain strain rate which is believed to pertain to the beta-transition. This transition is well-matched with a jump in the STZ activation energy at the same strain rate range; hence, the jump is referred to as the beta-transition activation energy, which is found to be about 10% of the STZ nucleation energy for PMMA.