The effects of amine donors ($a:NH_2,;b:NMe_2,;c:NMePh,;d:NPh_2$) and conjugation length on the molecular hyperpolarizabilities of a series of dipolar molecules have been theoretically investigated by using CPHF/6-31G method. The first hyperpolarizabilities (${eta}$) of p-nitrobenzene derivatives increase with the donor in the order, $NH_2;<;NMe_2;<;NMePh;<;NPh_2$, whereas slightly different order is observed in more conjugated derivatives, i.e., $NH_2;<;NPh_2;<;NMe_2;<;NMePh$. The result has been attributed to the extent of charge transfer and torsion angle. Moreover, the results show that "non-traditional" ${pi}$-conjugation effect exists in small compounds and decreases as the conjugation length between donor and acceptor increases.