The body-force effect on the cell formation of premixed flames at Lewis number unity is investigated by two-dimensional (2-D) and three-dimensional (3-D), unsteady calculations of reactive flows. To investigate the body-force effect on intrinsic instability, the relation between the growth rats and the wave number, i.e., the dispersion relation. is obtained. When premixed flames are propagated downward (upward), the growth rate decreases (increases) and the unstable range narrows (widens) with an increase in acceleration. Positive growth rates induced by hydrodynamic and body-force effects form a cellular flame. To investigate the cell formation due to intrinsic instability, the disturbance with the linearly most unstable wavelength is superimposed on a planar flame. The superimposed disturbance evolves, and eventually a cellular flame front is formed. When the flame is propagated downward, the spacing between cells is almost constant and the cell depth becomes smaller as the acceleration increases. When the flame is propagated upward, on the other hand, the former becomes smaller and the latter becomes larger. In addition, the cell depth and flame-surface area of 3-D dames are larger than those of 2-D flames. This is caused by the difference in spacing between cells and disposition of cells. (C) 2001 by The Combustion Institute.