Tobacco caterpillar (Spodoptera litura (Fabricius)) and oriental tobacco budworm (Helicoverpa assulta (Guenée)) are important insect pests on tobacco which causing serious damage by defoliation. The niches and relationships of the two species were studied in this paper. The results indicated that the spatial niche breadth value and spatial niche overlap value of S.litura were higher than that of H.assulta, the spatial niche similarity between them was 0.0973. The temporal niche breadth values and temporal niche overlap values of S.litura and of H.assulta were very similar, and the temporal niche similarity between them was 0.9485. The coexistence ratios of S.litura and H.assulta on upper leaves, mid-leaves and underneath leaves of tobacco plants were only 6.11%, 3.23% and 0.51%, respectively. The results may suggest that the competition between S.litura and H.assulta is extremely improbable. The two species can occur and damage on the same tobacco plant at the same time because they incline to choose different spatial vertical resources.