During the last decades, in Vietnam, studies on phosphorus fertilizer are mainly focused on rice but not for other crops which likely consume P as high as rice does. Additionally, the methodology applied for these studies usually are conventional. It seems that in Vietnam nuclear as well as isotope techniques are little applied in such a kind of studies like biological nitrogen fixation, nitrogen- and phosphorus-fertilizer use efficiency. However, these approaches have been proven to be powerfull tools for the above mentioned researches [IAEA TECDOC-875, 1996]. P-fertilizer use efficiency of tomato planted on Fluvisols and Hapli-Plinthic Acrisols, the two soil types which are popular for vegetable produce in Hanoi area, have been studied using the 32P labelled technique. The experiments have been conducted in pot scale with 5 kg soil in each pot. Single superphosphate labelled with 32P was used and applied to the soils at a rate of N:P:K as 70:50:20 kg per hecta. Tomato variety used in this study is of high yield and could be cultivated around the year in the Hanoi area. Tomato after planted was left on the open air and taken care with a watering regime similar to that used to apply by the local farmers. Appropriate agrochemical formulation could be applied to protect the plant from fungi and insect-pest if any. After around 4 months of transplanting the plant was harvested all the part above the ground, carefully washed off and air dried. Afterward, the plant samples were cut into small pieces, dried at 60oC untill unchange weight was obtained. P concentration in soils and in superphosphate, and in plant samples was analyzed by spectrophotometry using molibdate-vanadate method. Specific activity of 32P in plant and fertilizer samples was analyzed by liquid sciltillation counting on a LSC Aloka 2110, Japan. To do this, plant samples were ashed at 500oC for 4 hours then the ash retained was dissolved in HCl 2M for further counting for 32P. However, for fertilizer, the sample was dissolved in 2N HCl then washed and filterred repeatedly to ensure all the P content was dissolved then count for 32P activity on the same LSC. The quantities for P-fertilizer use efficiency estimation were % Pdff (percent of P-fertilizer derived from fertilizer) and % fertilizer P utilization. The former quantity shows how much the portion of the total P in plant was taken from fertilizer and the latter quantity shows how many P in plant was uptaken from the P applied fertilizer. From the experimental results obtained some conclusions can be drawn as follows: 1) Acrisols is not fertile, therefore to maintain high yield of agricultural produce fertilizers (N-P-K as well as microelements) should be applied. However, the rate of the fertilizer application to soils, in common and to poor fertile soils in particular, has to be studied in order to avoid the excess of nutrition which could lead to eutrophication in the aquatic environment. And it seems that isotope techniques could assist to solve this issue similarly it does in the fertilizer use efficiency studies 2) Due to it's light texture, Acrisols appears to facilitate tomato to more effectively uptake P from fertilizer so that %Pdff from Acrisols is higher than that from Fluvisols. 3) % Pdff of tomato on the Fluvisols and Acrisols was, respectively, 10,4% and 18.8% while % fertilizer P utilization of the plant from the soils was 2.61% and 4.15%, respectively. It appears that the % fertilizer P utilization of tomato on Fluvisols is almost 1.5 times lower as compared to that for tomato on Acrisols. This implies that the former soil appears to have higher capacity to immobilize P than the latter does. This was proven by a fact that the clay content of Fluvisols is higher than that of Acrisols. Authors: Tran Kong Tau1, Dang Duc Nhan2, Nguyen Van Toan3; Nguyen Thi Mai Hoa4. 1+4 Faculty of the Environmental Sciences, Vietnam National University, Hanoi. 2+3 Institute for Nuclear Science and Technology, Vietnam.