We exploit flow-Field Flow Fractionation (flow-FFF), equipped with on-line multi-angle light scattering (MALS) and UV-Vis photodiode array, to separate single wall carbon nanotubes (SWNTs) by length. Clear separations are demonstrated for both unfractionated SWNTs and SWNT fractions made by use of size exclusion chromatography (SEC). Rod lengths of the SWNTs are calculated from MALS and those are compared to rod lengths from hydrodynamic models based on latex sphere calibrations. Flow-FFF is capable of probing subtle differences in SWNT size distributions, which suggests that it may be a simple method of SWNT identification and quality assessment.