The synthesis of poly(acrylonitrile-butadiene-styrene) (ABS) was carried out using the mass-suspension process, high cis polybutadiene (PB), benzoyl peroxide (BPO) as the initiator and modified and unmodified nanoparticles of zinc oxide (nano-ZnO). X-ray diffraction (XRD) and Fourier transform infrared (FTIR) were used to characterize the ZnO nanoparticles. A Universal tensiometer, Brokfield viscosimeter and transmission electronic microscopy (TEM) were used to characterize ABS. The employment of nano-ZnO induced an impressive morphologic modification and transformed the highly occluded salame morphology of poly(styrene-acrylonitrile) (SAN) to a highly dispersed one in its matrix. During the reaction, a decrease in the total production of SAN was observed due to the interaction between radicals produced by the initiator and nano-ZnO, causing a molecular weight increase of SAN. The nano-ZnO also induced a decline in the phase inversion and extended the period of occurrence. ABS without nano-ZnO yielded higher mechanical properties than one without any nanoparticles.