By using time-resolved electric-field-induced optical second-harmonic generation (EFISHG) measurements, we directly probed carrier transients in double layer (α-NPD/Alq3) organic light emitting diodes (OLED) (α-NPD, N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine; Alq3, tris(8-hydroxy-quinolinato)aluminum(III)). Results showed that carrier transients are comprised of two relaxation processes. One is charging on electrodes in a single exponential form exp(−t/τRC) with a time constant of τRC = RsC0 (Rs, connecting lead resistance; C0, device capacitance), independent of applied voltage. The other one is charging at the α-NPD/Alq3 interface in a stretched form exp(−(t/τ)β) (β = 1.6 ± 0.1) with a relaxation time, τ, proportional to V−2.1 (OLED with Al cathode) and V−2.6 (LiF/Al cathode). The Maxwell−Wagner model analysis well accounts for the two relaxation processes. We conclude that analysis of the carrier transients, directly probed by EFISHG, is effective for understanding the carrier me...