The water-fine SiO2 particle suspension with a narrow-size distribution (mean diameter of 2.97 mum, standard deviation of 0.03 mum) was heated in a rectangular vessel from a vertical wall and cooled from the opposing vertical wall. Temperature distribution and local particle concentration of the suspension were measured under various temperature differences between the opposing vertical walls. It was found that behaviors of natural convection in the suspension were classified into five patterns. The critical wall temperature difference that would give rise to change of the natural convection pattern decreased as the initial particle concentration decreased.