The unburned carbon in fly ash is one of the important factors for the boiler combustion condition. Controlling the unburned carbon in fly ash is beneficial for fly ash recycle and to improve the combustion efficiency of the coal. Laser-induced breakdown spectroscopy (LIBS) technology has been applied to measure the fly ash contents due to its merits of non-contact, fast response, high sensitivity, and real-time measurement. In this study, experimental measurements have been adopted for fly ash flows with the surrounding gases of N-2 and CO2, while the CO2 concentration varified to evaluate the CO2 effect on the unburned carbon signal from fly ash powder. Two kinds of pulse width lasers, 6 ns and 1 ns, were separately adopted to compare the influence of laser pulse width. Results showed that compared with that using 6 ns pulse width laser, plasma temperature was lower and had less dependence on delay time when using 1 ns pulse width laser, and spectra had more stable background. By using 1 ns pulse width laser, the emission signal from surrounding CO2 also decreased because of the less surrounding gas breakdown. The solid powder breakdown signals also became more stable when using 1 ns pulse width laser. It is demonstrated that 1 ns pulse width laser has the merits for fly ash flow measurement using LIBS. (C) 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.