Abstract: Does tree root affect crop growth when they are planted near by each other? In aspect of allelopathy, the paper gives results based on the study in Agro-ecological research station of Feng Qiu in He Nan province. In a general survey, it was found that model of intercroping included corn with Paulownia elongata, wheat growing in orchard of Malus pumila, sweet potato planted in orchard of Prunus persica, and various crops growing near by the tree of Populus Canadensis in surveyed surroundings. Under normal climatic and nutrient condition, it did not occur that those crops grew apparently poor except sweet potato which gave bitterish taste in comparison with other sweet potato growing not near to Prunus persica. Results show that water extracts of tree roots inhibited germination and early growth of wheat. The germination inhibitory extents were in the order that by root of Prunus persica>by root of Populus canadensis>by root of Malus pumila>by root of Paulownia elongata. The shoot growth and root elongation of wheat were also affected by the water extract of tree roots. Populus canadensis was the strongest one in the three sorts of the trees, and Paulownia elongata was the weakest one. Ether extract of tree roots concentrated some organic compounds from the water extracts and exhibited more obvious inhibitory effect on wheat growth. The order of inhibitory extents by ether extracts was the same as that by the water extracts. From literature,it is known that in soil or water phlonzin from apple root bark will be degraded into phloretin,P-hydroxyhydrocinnamic acid ,P-hydroxyhenzoic acid and phloroglucinol, gradually. An experiment by using the commercial chemicals of P-hydroxybenzoic acid and phloroglucinol was set up in order to answer if the compounds played role in the inhibitory effect of tree root . The results of experiment show that the water solution of organic compounds did take part in the inhibitory effect on the germination and early growth of wheat. At concentration of n × 10-2M,it showed inhibitory effect , but at n × 10-5M,it showed simulative effect. P-hydroxybenzoic acid was detected in the soil samples collected from the field station. The concentration of the compounds varied around the level of n × 10-5M. The paper elucidated the condition under which inverse effects occur.