The existence of an indentation size effect (ISE) in the measured hardness in poly (methyl methacrylate) is investigated by performing nanoindentation experiments. The continuous stiffness measurement technique was employed to measure the hardness and the elastic modulus of the sample as a function of the indentation depth at different strain rates. Results establish the existence of a strain rate dependent ISE in the glassy polymer as hardness increases with decreasing the indentation depth. Employing the well-known shear transformation (ST) mediated flow theory and considering the statistical nature of the formation and distribution of the flow units, a rate dependent ISE model for glassy polymers is developed. This model is based on the possibility of the occurrence of discrete shear transformation sites within the deformed volume under the indenter which is controlled by the indentation depth and geometry. Results obtained from the proposed model show good agreement with the experimental values. The details considered in the model development in terms of the variations of indentation strain rate and elastic modulus with depth as well as validity and limitations of the model are discussed. (c) 2018 Elsevier Ltd. All rights reserved.