The effect of change in concentration of messenger molecule inositol 1,4,5-trisphosphate （IP3） on intracellular Ca^2＋spiral pattern evolution is studied numerically. The results indicate that when the IP3 concentration decreases from 0.27 μM, a physiologically reasonable value, to different values, the spiral centre drifts to the edge of the medium and disappears for a small enough IP3 concentration. The instability of spiral pattern can be understood in terms of excitability-change controlled by the IP3 concentration. On the other hand, when the IP3 concentration increases from 0.27 μM, a homogeneous area with a high Ca^2＋ concentration emerges and competes with the spiral pattern. A high enough IP3 concentration can lead the homogeneous area to occupy the whole medium. The instability of spiral pattern is ascribed to the change in stability of a stationary state with a high Ca^2＋ concentration.
