The ductility of a calcium carbonate-filled amorphous copolyester PETG in a uniaxial tensile test was examined as a function of the filler volume fraction. A ductile-to-quasi-brittle transition occurred as the volume fraction of filler increased. This transition was from propagation of a stable neck through the entire gauge length of the specimen to fracture in the neck without propagation. The draw stress (lower yield stress) did not depend on the filler content and was equal to the draw stress of the unfilled polymer. It was therefore possible to use a simple model to predict the dependence of the fracture strain on the filler volume fraction. It was proposed that when the fracture strain decreases to the draw strain of the polymer the fracture mechanism changes and the fracture strain drops sharply. The critical filler content at which the fracture mode changes is determined primarily by the degree of strain-hardening of the polymer.