The stability of nano-particle suspension is researched in the paper. Experiments are performed for the SiO2 nano-particle suspensions with different base fluids to observe their stability. On the basis of the experiment, the factors affecting the stability namely the equivalent diameter of the nano-particles and the dynamic viscosity of the base fluid are discussed and some valuable conclusions are obtained. It can be seen that the most important factor to affect the stability of nano-particle suspension is the dynamic viscosity.