The reliability index is usually utilized for structural design problems. Relations between design results, such as configurations, and the reliability indices in accordance with the Advanced First Order Second Moment (AFOSM) method, are numerically investigated with the help of some fundamental problems and practical examples. In general, the AFOSM method contains some intrinsic errors. When the index value is large the error in the failure probability increases significantly, even if the error in the index is relatively small. It is explained that the design result is not affected to the same degree as the error in the failure probability by the error in the index, but it is usually affected to the same or somewhat larger degree as the error in the index. It is preferable to use the reliability index rather than the probability by which the design results are indirectly affected.