A diagram which not only considers the transition of the structural type hut also describes the rule of change of Tc in the A-15 compounds is given in this paper. From this diagram it can be seen that compounds with very high Tc(>25K) are not likely to exist in the A-15 series considered. In the Nb series, Nb3Ge probably has the highest Tc; in V series, the Tc can still be expected to be raised (>17.1K), the Tc of V3Ge is likely to be raised to above 12 K. Examining the case of Nb3Si, one can not get the optimistic estimation as that of Geller. It is considered that Nb3Si of A-15 type obtained by present new technology is a non-ideal structure formed at the expense of the normal stoichiometric ratio or by a change of the ordinary atomic volume. On the basis of the relationship between Tc and radii ratio and on consideration of the radii ratio required by the ideal A-15 structure, the point of view presented in this paper does not agree with the various predictions that the Nb3Si might have a high Tc(>23 K).