To clarify the relationship between secondary electron emission coefficient and secondary ionisation coefficient, Ar dielectric barrier discharges have been simulated using one dimensional fluid model. The Paschen's curves are obtained and secondary ionisation coefficients are derived from the Townsend's breakdown criterion. Each secondary electron emission coefficients affect to breakdown voltages and secondary ionisation coefficients. However, it is found that the secondary ionisation coefficients are obtained in similar to that of secondary electron emission coefficients.