Abstract: When urea was the only source of nitrogen for hydroponic culture of vegetables,1.under controlled temperature ,no significant transformation of ruea was observed in the deionized water-made nutrient solution when no crops were grown ,but significant one was presented in the same solution after growing vegetables.The higher was the temperature,the faster was the urea transformation.The solution's pH was ranged between 5.0-6.3 during this period;2.under room temperature,the urea was significantly transformed and its ammonification and nitrification were almostly presented at the same time in the nutrient solution made by tap water or deionized water,respectively.At the end fo the 3rd week,about 90%of supplied urea-N was transformed into MM4+-N and NO3--N,each one was accounted for about one half of the transformed urea while the pH of both nutrient solutions were ranged between 4.2-6.9.when lettuce (Lactuca saliva L.) and water spanich (Ipoinoea aquatica) were respectively grown in the nutrient solution with urea as its only N constituent,1.no undesirable response of water spanich was obtained,but damaged symptom was appeared in the lettuce body;2.the pH of solution could be dropped down to below 4.0;3.the urease activity in the roots of water spanich was lower than that of lettuce.