Safety factor of 1.5 was specified in paragraph 25.303 in China Civil Aviation Regulations(CCAR)Part 25.This requirement and other relevant paragraphs in CCAR25, such as CCAR25.307"Proof of Structure",25.603"Materials",25.605"Fabrication methods",25.613"Material strength properties and material design values", ensure the reliability of airplane structure collectively.Theoretically, when the uncertainty for material properties, load and analysis reduces, the application of safety factor less than 1.5 can ensure equivalent reliability for airplane structure. In other hand, if the uncertainty for material properties, load and analysis exceed the allowable level implied in current airworthiness standards, application of safety factor greater than 1.5 may still ensure equivalent reliability for airplane structure. To determine proper safety factor for certain certification circumstance without reduction in the structure reliability, the composition of safety factor 1.5 was researched. Based on the investigation of the relationship among the relevant paragraphs of China Civil Aviation Regulations (CCAR) Part 25, the safety factor of 1.5 specified in CCAR 25.303 is divided into three sub-factors. The sub-factor for load uncertainty is determined to be 1. 2; the sub-factor for material properties uncertainty is determined to be 1.12; the sub-factor for analysis methods uncertainty is determined to be 1.116.