An experimental study has been performed on the concentration fluctuation during atmospheric diffusion of hydrogen released horizontally from 20 or 40 MPa pressurized storage. The aperture of nozzle is selected to a pinhole of d = 0.2 or 0.53 mm diameter. The concentration fluctuation with time was measured with needle sampling and fast response analyzing device, and the probability density function (pdf) of hydrogen concentration was approximated with a normal distribution function and the time-averaged concentration along the axis of jet could be expressed with C = alpha/(x/theta) where alpha is proportional constant, x is the distance from nozzle outlet, and theta is the nozzle diameter corrected by the square root of the ratio of atmospheric to stagnation pressure in nozzle. Furthermore, the spectrum of concentration fluctuation was analyzed to decay with -5/3 power of wave number except in the vicinity of nozzle outlet, and the typical characteristic spatial scales of the concentration fluctuation at the beginning of the inertial convective subrange were about 100 and 200 mm for d = 0.2 and 0.53 mm, respectively. The probability of ignition by using an electric spark and the behavior of flame propagation were examined, and for C < 10%, the flame was once formed and extinguished after a short travel of approximately 100 or 200 mm, which may correspond to the characteristic spatial scale of eddy or inhomogeneous hydrogen distribution. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.