Abstract: Since LED is a convenient transfer standard, it is widely used in metrology. As a standard, the value of LED should be accurate and stable over a long period of time. LED is selected as standard because its luminous intensity distribution curve（LIDC）is close to that of Lambert light source. The concept of LIDC and the distribution photometer for measuring LIDC are introduced. Three typical electric light source options are given. Ten typical LIDCs and three erroneous LIDCs are given when LED is used as standard. The reason for selecting white light LED with good emitting feature as standard LED is elaborated.