Abstract: Although oil rape (Brassica napus) is generally sensitive to P deficiency,there are still extensive genotypic differences between various cultivars. So, research on its physiological mechanism is helpful to instruct production of rape. Effect of phosphorus on root parameters, inorganic P and acid phosphatase (APase) of two typical rape cultivars 97029 and 97081 were studied under solution culture. Results showed that efficient cultivar 97081 has 9.3%,21.9%,10.9%,7.8% of higher main root length, root volume ,root/top and root activity than inefficient 97029 under P deficiency. That means the root conformation of 97081 is available to absorb more P from soil than 97029.With the lowering of P in medium, inorganic P would decrease but APase activity increase in each organ of both cultivars. However, significant genotypic differences of them can still be found, under same P supply,97081 can keep much higher inorganic P, especially in new leaves; as far as the activity of APase is concerned,97081 changed in a smaller range. That is to say, root of 97081 weakened more slowly to keep better absorption capability and more potential for seed yield, but 97029 couldn't.