This paper presents the performance of dynamic light scattering (DLS) and differential mobility analyzer (DMA) instruments in measuring nanoparticle size data using a gauge repeatability and reproducibility (GR&R) study based on the mixed model. The adequacy of the measurement process was evaluated using a measure of %GR&R with its confidence interval. The results show that the performance of the DMA instrument is superior to that of the DLS with regard to all variance components of a GR&R study. However, both instruments performed poorly in terms of %GR&R. Therefore, the part and measurement system require further improvement. Using these empirical study applications, this research promotes and maintains measurement quality through procedure analysis.