Plastic deformation behavior and the features of internal shear bands in Pd79Cu4Au2Si10P5 bulk metallic glasses were investigated. The results show that the samples with diameters of 1 mm, 2 mm and 3 mm all have lager plastic deformation and the plastic strain is about 13%, 10% and 8%, respectively. In addition, the samples with different diameters have distinctly different serrated flows on the engineering stress-strain curves. The serrated-flow amplitude of the specimen with diameter of 2 mm is the largest, and the smallest serrated-flow amplitude is obtained in the specimen with diameter of 3 mm. The serrated-flow space of the specimens with diameters of 1 and 3 mm is smaller than that of the specimen with diameter of 2 mm. The results observed by optical microscopy show that, with the increase of sample size, the density of the internal primary shear bands decreases, but the density of the secondary shear bands increases. Further studies by scanning electron microscopy show that the thickness of primary shear band increases with the increase of sample size