Abstract: This paper deals with the precipitation behaviour of Mo and Zr which depends on the concentrations of Mo,Zr,nitric acid and temperature. Precipitation,post-precipitation and ultracentrifugation experiments are made at 100℃ , 80℃ ,60℃ ,40℃ and room temperatures in the range of 0. 6-6. 0mol/ 1 nitric acid.The experimental feeds are made up of molybdenum labelled with 99Mo,zirconium labelled with 95Zr and nitric acid solution. The feed is allowed to stand at constant temperature for some time for the observation of precipitation behaviour. The filtered precipitate and ultracentrifuged liquid is to be measured with HP (Ge)-multichannel analyser in order to determine the content of Mo,Zr and their mole ratio in the precipitate and to find out whether there is colloid in the liquid.The results show that the mixed solution of Mo and Zr can produce precipitate and post-precipitate in nitric acid. If the filtrated liquid is allowed to stand for some time precipitate can be produced again, until the concentration of Mo and Zr in the feed is too low to form precipitate, such as 2. 5×10-3 mol/l . If the concentration of nitric acid is less than 4. 0mol/l ,the precipitation is produced easily and more precipitate is formed. Precipitation is slower in solutions which are more than 4. 0mol/l in HNO3. The mole-ratio of Mo to Zr in the precipitate is 2 to 1 and it is not dependent on that ratio in the system.