Instability of a horizontal porous medium layer saturated by a nanofluid heated from below is revisited. The study employs Buongiorno’s model for nanofluids and Darcy model for porous media. Different from the previous studies, this analysis is performed based on a nonlinear base-state solution, obtained by considering the dependence of thermophoretic diffusion coefficient on the volume fraction of nanoparticles. It is found that an infinitesimal temperature gradient is sufficient to overcome Darcy drag and trigger the onset of convection in a porous layer. In comparison with the previous works, the threshold of instability in presence of the nonlinear particle distribution shifts to lower concentration by two orders of magnitude. The physical mechanism causing the enhancement of instability is explained via an order of magnitude analysis. The theoretical result suggests that the instability in a porous media can be enhanced substantially by suspending nanoparticles.