We have determined the first several coefficients of the series formula (1) for superconducting Tc. For the double-σ spectrum as α2F(ω)=λω/2[a1δ(ω-ω1)+(1-a1)δ(ω-ω2)] and that of several real materials, we have compared the Tc calculated from the series formula, the Allen-Dynes formula and the numerical solution of the Eliashberg Equation, respectively. The results suggest that when the series is convergent, the calculated results by using the series formula is a better approximation of the numerrieal solution than that from the A-D formula.We have also given an approximate Tc series formula, obtained a method to estimate the convergent radius of the Tc series and calculated the convergent radius for several materials. Therefore we may estimate the region in which the series formula (1) is valid.