Most of the particle number emitted by modern diesel engines equipped with diesel particle filters (DPF) consist of volatile nanoparticles smaller than about 30 nm dia. Volatile particle number and size are significantly influenced by the dilution process. A dilution process that is good for a cold climate region may not be good for an area experiencing warm climate conditions. However, there is at least a need to carefully craft a measurement protocol to harmonize particle number and size measurement for laboratory testing. The particle number and size measurement protocol should not only include the sampling method but also should address the traceability, accuracy, linearity, and calibration of the particle instruments used, in addition to avoiding particle losses in the sample train using good engineering aerosol measurement practices. Harmonizing the protocol of particle number and size measurement may enable the stake holders to capture all the metrics of interest such as number, size, derived surface area, and derived volume and mass at a very low concentration level. This is an abstract of a paper presented at the 102nd Air & Waste Management Association Annual Conference and Exhibition (Detroit, MI 6/16-19/2009).