Abstract: The concept of safety factor with two-indexes of reliability and confidence levels was presented by introducing the confidence level index.Based on the reliability theory,new formulae of safety factor with two-indexes of reliability and confidence levels were derived to take the sample size of experimental data into account and to determine the safety factor value from small sample test data for structural reliability design.The new safety factor was applied to two sets of experimental results,demonstrating the practical and convenient use of the proposed technique.It is shown from these examples that the safe factor can be obtained realistically according to small sample test data using the new formulae.