In this work we present a comparison exercise between two metrological institutes for size measurement of silica nanoparticles by Asymmetrical Flow Field-Flow Fractionation (AF4) coupled to static light scattering. The work has been performed in the frame of a French inter-laboratory comparison (ILC) exercise organized by the nanoMetrology Club (CnM). The general aim of this multi-technique comparison was to improve the measurement process for each technique, after establishing a well-defined measurement procedure. The results obtained by two national metrological institutes (NMIs), the LNE (France) and the SMD (Belgium) by AF4-UV-DRI-MALS will be presented and discussed. Three different samples were characterized: the reference material ERM®-FD304, which is a suspension of colloidal silica in aqueous solution and two silica bimodal samples consisting of two populations of SiO nanoparticles of unknown size in aqueous solution, with different populations’ ratios. The procedure for the preparation of the sample before the analysis, and main separation parameters have been previously defined between the two institutes and will be described. The principals measured parameters were the weight-average (d), number-average (d) and z-average (d) geometric diameter; the average hydrodynamic diameter (d); and the diameter obtained by external calibration using polystyrene latex standards (d). Results between the two NMIs were comparable and coherent with the expected size values of those obtained by other techniques like Scanning Mobility Particle Sizer (SMPS) and Scanning Electron Microscopy (SEM) also involved in this ILC exercise. Where discrepancies are observed, they leave the results compatible within their uncertainties and underpin the challenges in analysing data and reporting results, making AF4 a powerful tool to compare to other measurement techniques.