In resent years, a large number of fast-growing exotic species had been introduced to south China for restoration and recon-struction of degraded ecosystem and economic development. Eucalyptus grandis, one of important tree species, was widely used in the Project of Returning Farmland to Forest and also used in afforestation in mountainous area. With the rapid development of E. grandis plantation, It has been not only brought tremendous economic benefits but also caused a series of ecological and environmental problems, such as soil degradation, the loss of soil nutrients, the decrease of soil productivity and the biological diversity shrinking, so its develop-ment has been attracted a great deal of controversy. It was reported that E. grandis could release certain chemical substances to inhibit the growth of the plants near and/or under the crown of E. grandis plantations. It is of great practical significance to explore the allelopathic effects and mechanisms of E. grandis litter(fallen leaves and dead roots) on some crops which were often intercropped with E. grandis in the system of agroforestry. Cichorium intybus was a kind of high-quality forage grass which was very common in south China, thus it was selected in this study.A pot experiment was conducted to study the effects of E. grandis root on the growth and the photosynthesis of C. intybus during its early stage of decomposition. Each pot contained 10 kg soil mixed with different amounts of E. grandis root (50 g/pot, A₁; 100 g/pot, A₂; and 0 g/pot, CK), and sowed with C. intybus. The growth indicators and the photosynthetic characteristics of C. intybus were measured after the third leaf of seedling fully expanded in treatment A₂. The results were as follows. At the early stage of E. grandis root decomposition, the height, root and leaf area growth, biomass accumulation, and the content of photosynthetic pigment of C. intybus were significantly inhibited, and the inhibition effect was getting stronger with the increasing amount of E. grandis root addition; The intercellular CO₂ concentration(C_i) of C. intybus was increased by root addition, while net photosynthetic rate(P_n), stomatal conduct-ance(G_s) and transpiration rate(T_r) were significantly lower than those of the control; With the increase of root addition, all parameters of light response and CO₂ response of C. intybus showed an obviously downward trend except CO₂ compensation point, and there existed significant differences between the treatments of root additions and the control; The growth indexes showed a negative correlation with the intercellular CO₂ concentration, while it showed significant or extremely significant positive correlation with other photosynthetic parameters, photosynthetic pigments, and the corresponding parameters of the response curve; Detection wtith GC-MS showed that E. grandis, roots contained chemical substances which had allelopathic potential, such as 2,6-Bis(1,1-dimethylethyl)-4-methylphenol and Benzeneeth-anamine, N-methl. It was indicated that, when E. grandis root decomposed, the allelopathic substances released gradually and acted on receptor plants, inhibited the synthesis of photosynthetic pigments and the photosynthesis of the receptors, decreased the adaptation ability of receptors to environment and accordingly, inhibited the growth of C. intybus.