In recent years, with the development of the semiconductor manufacturing process, ultrapure water (UPW) used to wash semiconductor devices have required high purity. In particular, particles in UPW are strictly controlled. Our 10 nm SEM analysis can measure 10 nm particles, which was previously impossible. This article presents the results of measuring 10 nm particles in the existing ultrapure water system. We clarified the behaviour (including generation sources and compositions) of 10 nm particles in ultrapure water systems in order to optimize and enhance. This article was originally published in the Ultrapure Micro Journal in November 2017.