The tensile properties of polypropylene (PP) ®lled with two A-glass beads with the same size, PP/3000 (glass bead surface pretreated with a silane coupling agent) and PP/3000U (no surface pretreatment), have been measured by using an Instron materials testing machine at room temperature, to identify the effects of the ®ller surface pretreatment and its content on the tensile properties of these composites. The results show that the Young's modulus E c of the composites increases non-linearly with increasing volume fraction of glass beads f f , while the tensile yield strength s yc and tensile stress at break s bc of the composites decrease with an increase of f f , in the f f range 0± 30%. Furthermore, the values of E c and s bc of the PP/3000 system are somewhat higher than those of the PP/3000U system under the same test conditions, but this is in contrast to the tensile strain at break ε ε ε ε ε ε bc and tensile fracture energy E bc , especially at higher f f values. Good agreement is shown between the measured tensile strength and the predicted value by using an equation proposed in previous work. In addition, ε ε ε ε ε ε bc and E bc reach maximum values at f f = 25% for both systems. This indicates that there is a brittle±ductile transition for the composites in tension.