A series of numerical simulations of premixed cylindrical expanding ethanol/air flames in a constant volume were performed to investigate the cellular instability. Then a theoretical analysis in terms of critical Peclet number

logarithmic growth rate and critical flame radius was carried out to explore the flame instability. The results demonstrate that at pressure of 1 MPa and temperature of 358 K

ethanol/air flame cellular instability increases non-monotonically with the equivalence ratio changing from 0.8 to 1.6 and has the most intense instability when the equivalence ratio is 1.2. It is found that thermal-diffusion instability changes dramatically with increased equivalence ratio mainly due to molecular diffusion

and logarithm growth rate increases first and then decreases. On the contrary

hydrodynamic instability is insensitive to the equivalence ratio. Finally

the almost constant critical Peclet number and remarkably reduced flame thickness lead to a great decrease in critical flame radius when the equivalence ratio is less than 1.2

and critical flame radius reaches the minimum when the equivalence ratio is 1.2. Both numerical and theoretical investigations showed a consistent result.