Abstract : Kerosene heaters are known to emit pollutants in the indoor environment but ultrafine particle emission was never reported. A kerosene heater was characterized in an experimental chamber using four different instruments to monitor particle number concentration and size distribution down to 5 nm. Another experiment was realized to collect particles and determine their composition in elements, ions, organic/elemental carbon and polycyclic aromatic hydrocarbons. The kerosene heater produced ultrafine particles in the range 5 to 40 nm with a count median diameter of 9.5 nm. The emission rate was (1.8±0.3)×1011 particle*s-1. Chromium and manganese (4 and 0.05 ng*g-1 of burned fuel) were specifically associated with kerosene combustion. Trace levels of sulfates, nitrates, phenanthrene and fluoranthene were also detected. Scanning electron microscopy showed submicronic aggregates mostly composed of carbon and traces of silicon. Our findings showed that kerosene heaters represent a major indoor source of ultrafine particles.