The particle number (PN) emission regulation has been implemented since 2011 in Europe. PN measurement procedure defined in ECE regulation No. 83 requires detecting only solid particles by eliminating volatile particles, the concentrations of which are highly influenced by dilution conditions, using a volatile particle remover (VPR). To measure PN concentration after the VPR, a particle number counter (PNC) which has detection threshold at a particle size of 23 nm is used, because most solid particles generated by automotive engines are considered to be larger than 23 nm. On the other hand, several studies have reported the existence of solid and volatile particles smaller than 23 nm in engine exhaust.This paper describes investigation into a measurement method for ultrafine PNCs with thresholds of below 23 nm and evaluation of the VPR performance for the particles in this size range. The detection efficiency of an ultrafine PNC was verified by following the ECE regulation procedure. In addition, the possibility of re-nucleation of volatile particles at the VPR outlet was evaluated, because the re-nucleated volatile particles which are usually smaller than 23 nm will cause a high bias for solid particle measurements by the ultrafine PNC compared to the standard PNC. We applied a hot catalytic stripper (HCS) in order to effectively remove volatile fractions instead of an evaporation tube (ET). The modified VPR with the HCS showed higher removal efficiency for volatile particles than the ET. However, technical challenges like higher losses of the nanoparticles were revealed by this investigation.