Ultrashort single-walled carbon nanotubes, i.e. with length below ~30 nm, display length-dependent physical, chemical and biological properties that are attractive for the development of novel nanodevices and nanomaterials. Whether fundamental or applicative, such developments require that ultrashort nanotube lengths can be routinely and reliably characterized with high statistical data for high-quality sample production. However, no methods currently fulfill these requirements. Here, we demonstrate that photothermal microscopy achieves fast and reliable optical single nanotube analysis down to ~10 nm lengths. Compared to atomic force microscopy, this method provides ultrashort nanotubes length distribution with high statistics, and neither requires specific sample preparation nor tip-dependent image analysis. Ultrashort single-walled carbon nanotubes (usCNTs) hold unique physical, chemical and biological properties that can be used for applications in diverse areas. In condensed matter physics, both theory and experiments have shown that the nanotube electronic band-gap increases as their length shortens down to tens of nanometers in response to quantum confinement effect along the length axisRead less <