Abstract: A method is described for the determination of the isotopic composition of uranium by activation analysis. After irradiation of samples containing a small quantity of uranium in reactor, the energy spectrum of 239Np formed from 238U and those of fission products formed from 235U are measured with high resolving power semiconductor-computer system. The ratio of the peak intensities is proportional to the 238U/235U ratio in the sample.A series of standared samples with 235U abundance ranging from 0.6% to 60% were irradiated in the heavy water reactor of the Institute of Atomic Energy at a neutron flux of 3×1013n cm-2 sec-1. The intensity ratios of the three 239Np photo peaks and nine fission product peaks were plotted against the 238U/235U ratio of the samples respectively. Straight lines of zero intercept were obtained in all cases using Ge (Li) detector. High purity Ge detector were used to determine the X-ray characteristic peaks of 239Pu and three fission products. Straight lines were also obtained when the X-ray characteristic intensity ratios of the 239Pu and those of some of the fission products were plotted against the 238U/235U ratio.These methods can be used for the analysis of samples containing 1-100/μg uranium with a precision of 1%-3% for samples with 235U abundence ranging from 0.6% to 55%.