The objective of the present study is to study the Soret effect of both nanoparticles and solute on the convective instabilities in binary nanofluids. A new stability criterion is obtained based on the linear stability theory. The results show that the Soret effect of solute(${psi}_{bf}$) makes the binary nanofluids unstable significantly and the convective motion in a binary nanofluid sets in easily as the ratio of Soret coefficient of nanofluid to that of binary basefluid ${delta}_4$ increases for ${delta}_4$ > -1. It is also found that as an increase of the volume fraction of nanoparticles, nanofluid becomes stable but at a separation ratio of ${psi}=-0.3$ the state of fluid changes from stable to unstable.