Second‐order hyperpolarizabilities have been calculated by the three‐level model for systems having low‐energy double transitions. In the three‐level model, the second excited state, in addition to the first excited and ground states, is explicitly taken into account. The second‐order hyperpolarizabilities calculated by the three‐level model gave excellent agreement with the observed values determined by the dc induced second harmonic generation technique, for 4‐aminostilbene,p‐nitroaniline, and 2‐methyl‐4‐nitroaniline.