Abstract: In order to evaluate the analysis and determination technique of naturally occurring radioisotopes in our lab for environmental samples, we participated in the IAEA 2008 world-wide open proficiency test on the determination of natural radionuclides in phosphogypsum and spiked water. This paper mainly focuses on the gross α/β analysis in spiked water. According to the requirement of IAEA, Sample03, Sample04 and Sample05 were tested for the gross α/β radioactivity. Comparative determination method was chosen, and UO₂(NO₃)₂ and KCl were selected as sub-standard and standard reference substances for gross α analysis and gross β analysis, respectively, by determining radioactivity with MPC 9604 ultra low level α/β counter. The feedback statistics from IAEA final report show that the values of gross α/β of all of the three samples fell in the acceptable range of IAEA. The influence factors of gross α/β analysis such as the sensitivity of the α/β counter, the selection of standard reference substances and how to monitor the background and efficiency for daily determination and so on were also discussed. The results indicate that when one selected O₂(NO₃)₂ as sub-tandard reference substance for gross α analysis, the total α activity of O₂(NO₃)₂ is much lower than that of standard U₃O₈ which is recommended by B 5750—85, just because the ²³⁸U and ²³⁴U do not reach the secular equilibrium for our lab sub-tandard O₂(NO₃)₂ eagent. But no effect of such UO₂(NO₃)₂ on the analysis of gross α activity is observed after activity of ²³⁴U and ²³⁸U are qualitative measured by α pectrometry.