Effect of internal heat source on the onset of convection in nanofluid layer heated from below is studied. The lower boundary and upper boundary are assumed to be rigid and free respectively. The two important effects namely the Brownian motion and thermophoresis have been included in the model of nanofluid. Linear stability analysis has been made to investigate the effect of internal heat source on the onset of convection. Galerkin method is used to obtain the analytical expression for Rayleigh number in the non-oscillatory mode and result are depicted graphically. It has been shown that the internal heat source, nanoparticle Rayleigh number and modified diffusivity ratio have a destabilizing effect depending upon the values of various nanofluid parameters.