The mechanisms of morphological and physiological plasticity of root system of seeding tomato cutivars under low-phosphrus stress were studied with solution culture. Results showed that low-phosphorus stress restricted shoot growth, but stimulated root growth. The different phosphorus efficiency of low-phosphorus-tolerant cultivars attributed to phosphorus uptake ability not the different distribution of phosphorus and utilization ability. The average length of lateral roots of low-phosphorus-tolerant tomato cultivars was longer than that of low-phosphorus-sensitive tomato cultivars under low- phosphorus stress, but the average number of lateral roots did not differ significantly among different tomato cultivars. The acid phosphatase activity of different cultivars rose significantly in low- phosphorus stress. In the normal phosphorus treatment, acid phosphatase activity of root was the lowest, but the increment range of acid phosphatase activity of low-phosphorus-tolerant tomato cultivars was higher than that of low-phosphorus-sensitive tomato cultivars under low- phosphorus stress. Low Km and Cmin were characterized by low-phosphorus-tolerant tomato cultivar under low- phosphorus stress. Key words: tomato; phosphate stress; acid phosphatase; root/shoot ration.