Experiments have been done on bi and tetra-crystal competitive growth in a nickel base single crystal superalloy to imitate the competitive growth during directional solidification. The grain orientations were controlled by seed crystals. Results show that the competitive growth is dependent on the relative orientations of grains and their relation with the direction of maximum temperature gradient. Grains with their preferred growth direction (<001> in nickel base superalloy) nearer to maximum temperature gradient will grow faster than those with their preferred growth direction far away from maximum temperature gradient in the condition of bi-crystal as well as tetra-crystal competitive growth. A third grain may appear and grow as a single crystal if two seed crystals with their preferred growth directions are far away from that of maximum temperature gradient in a divergent way in bicrystal competitive growth.