Abstract: The argument about applied safety factor in design of aircraft structures has been disputed for more than fifty years. At the beginning it was oniy considered to select a factor according to experience in practice to guarantee the safety. Some designers took five times as much as the applied load in structural design and test. Obviously this approch is not very reliable. The name' load factory' did not appear untill 1920s. In 1934 some Americans first took 1.5 as the safety factor, then people in other developed countries followed. However, as the aeronautical technology has been developing and various materials are being used, it is necessary that the safety factor be further discussed now.In general, there are two theories in this field. One is olassical, i. e. to take a fixed figure is 1.5 as safety factor. Another is based on probability. The former is simple and easy to ultilize, but not exact. The later is considered to be better, but more complicated and difficult, having to be varified by a great many of tests.This paper will discuss the classical theory of safety factor and make a comparison between the two theories, and then present a principle of selecting the safety factor. Finally, some examples and ideas to analyse the safety factor used in composite material structures will be given.