Particle coverage ratio on bubble interface was investigated by metallographic analysis when the foam is critically stable in the gas injection foaming process. It is found that the critical coverage ratio was almost the same under different conditions, such as different particle amount, particle size, or particle wettability. The particle coverage ratio is proposed as the criterion to judge the foam stability, and a criterion equation was deduced. Adsorption coefficient of particles, effective coverage ratio of a particle and critical coverage ratio on the bubble interface are stability influencing parameters of the criterion equation. Their determination methods were presented and examined by air injection foaming of A356/Al2O3 melt. The adsorption coefficient calculated by measuring cross-section of foam cell wall gave values agreeing with the experimental results. The effective coverage ratio, which is about 0.9 in this article, can be calculated by combining measurements on surface and on the cross-section of the foam cell wall. The critical coverage ratio, that is about 14% in this article, can be determined by measuring the cross-section as well as the surface of the foam cell wall.