The formation of cellular flames and the increase in flame velocity generated by intrinsic instability are studied by two-dimensional (2-D) and three-dimensional (3-D) unsteady calculations of reactive flows based on the compressible Navier-Stokes equation. We consider three basic types of phenomena to be responsible for the intrinsic instability of premixed flames, i.e., hydrodynamic, diffusive-thermal, and body-force instabilities. Cellular flames are generated by intrinsic instability, and thus the flame-surface area becomes larger and the flame velocity increases. The increment in flame velocity of 3-D flames is about twice that of 2-D flames, since the increment in flame-surface area of the former is about twice that of the latter. This relationship is due to the difference in the disposition of cells between 2-D and 3-D flames. When the Lewis number is lower than unity, i.e., when diffusive-thermal instability appears, the increment in flame velocity is larger than in the flame-surface area. This is because the increase in the local consumption rate of the unburned gas at a convex flame surface exceeds the decrease at a concave one. When the Lewis number is unity, i.e., when hydrodynamic and body-force instabilities are dominant, the flame velocity is nearly proportional to the flame-surface area, since the local consumption rate is nearly constant.