Bènard instability in nanofluids is investigated in this work. The combined behaviors of Brownian diffusion and thermophoresis of nanoparticles reduce the critical Rayleigh number by as much as two orders
of magnitude. Turbulence is thus the dominating mechanism for natural
convection in nanofluids, which would promote heat transfer more than the
sole increase of the effective thermal conductivity alone. A nondimensional analysis is performed to extract the dominating parameters in transition from the laminar to turbulent regime. Analytical solutions for the critical Rayleigh number are derived to realize the effects of Brownian diffusion and thermophoretic motion of nanoparticles.