The effect of electron injection in polymer light- emitting diodes based on the phosphorescent material of fac- tris( 2- phenylpyridine) iridium ( Ir( ppy) 3) devices with CsF= MgAg= Ag cathodes were investigated. The device with MgAg ( 1 nm)= CsF ( 3 nm)= Ag cathode showed almost similar current density- voltage- luminance characteristics to that with CsF ( 3 nm)= MgAg ( 1 nm)= Ag cathode. The existence of interface between the organic layer and Cs or CsF results in low turn on voltage. To achieve the efficient electron injection and the low turn on voltage, it is necessary to exist MgAg and CsF interface at the position of about 1 nm from organic layer. To achieve the efficient electron injection, it is necessary to exist Cs layer just on the organic layer. The electron injection effects between devices with CsF and Cs are different. It is suggested that the dipole of CsF affect the efficient electron injection.