This paper is concerned with the reliability index of the second moment method for structural reliability analysis. The first problem considered is whether μ/σ
z is invariant, where μ
z and σ
2z are the mean and variance of the function z=G(X
1, X
2, …, X
n) and G≦0 represents the failure state. X
i(i=1n) are the failure governing random variables. It is shown that μ
z/σ
z is not invariant. This result reveals the essential cause as being that β
FOSM lacks invariance, where β
FOSM is the reliability index of the FOSM (First-Order-Second-Moment) method. Hence, if invariance is the indispensable requirement for the measure of reliability, μ
z/σ
z is not adequate as a basis of such measure. The second problem considered is the relation between β
AFOSM and μ
z/σ
z, where β
AFOSM is the reliability index of the AFOSM (Advanced-First-Order-Second-Moment) method. Since β
AFOSM is invariant whereas μ
z/σ
z is not invariant, β
AFOSM may not be a good approximation of μ
z/σ
z . Numerical comparison of μ
z/σ
z, β
FOSM, β
AFOSM< reveals that β
AFOSM is not a better approximation of μ
z/σ
z than β
FOSM is. Hence, β
AFOSM is better interpretted as the shortest distance from the origin to the surface G(X
1, X
2, …X
n)=0 in the space of the normalized variables than as an approximate value of μ
z/σ
z.
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