The effect of small heterogeneous grains with different features and shapes on the evolution of spiral wave is investigated by using the simple model of discrete excitable medium. When the small grains are uniformly distributed in a medium,the instability of spiral wave relates to the shape,density and nature of the grain. The numerical results show that under suitable conditions,the heterogeneous grains can lead to the transition from stable spiral wave to meandering spiral wave in the medium in which it occurs, the spiral wave even drifts out of the boundary of system. The periodic pattern with irregular shape and the periodic maze-type pattern have been observed for the first time. The physical mechanism underlying these phenomena is discussed.