Oligophagous Helicoverpa assulta (Guenée) and polyphagous H. armigera (Hübner) are two sibling species. Among their host plants, tobacco is the only one they share and coexist on. In order to further understand the ability difference in their olfactory orientation to tobacco volatiles, electroantennograms (EAG) were recorded from virgin and mated females, as well as males, of the two sibling species in responses to 20 tobacco volatile compounds at a low concentration (1.0×10-5 mol·L-1), and EAG dose-response curves were also constructed for 6 compounds. The results showed that: (1) Both H. assulta and H. armigera elicited similar EAG responses to the tested compounds, i.e., both species were more sensitive to the general odor componds, such as green leaf volatiles, aliphatic and aromatic compounds, than to the specific odor components of tobacco, such as monoterpenes, sesquiterpenes and heterocyclic compounds, suggesting that both species have the weak capability in distiguishing specific plant species from a distance. (2) The EAG responses of both insect species were maximal to the C6 aliphatic compounds, and declined as the carbon chain-length of compounds increased or decreased. (3) H. assulta elicited higher EAG responses to the most tested compounds than H. armigera, indicating that the olfactory sensilla of H. assulta are more sensitive to the host volatile compounds than H. armigera in their host orientation from a long distance. (4) In general, there were no significant sexual differences in their EAGs of the two sibling species, but the mated females of H. assulta elicited stronger EAG responses to β-phellandrene and sabinene hydrate than virgin females or males, implying that β-phellandrene and sabinene hydrate might be the ovipositional semiochemicals of H. assulta on tobacco plants. (5) The result of EAG dose-response curves indicated that EAG responses of both insect species were dose-dependent to 6 types of compounds in the tested concentration range, far from their maximum responses. In conclusion, these results indicated that the olfactory neural system of H. assulta is more specialized and plays more important roles in host orientation behaviors than that of H. armigera.