This paper presents 2D simulations of the ignition of a lean premixed H-2-air flame by a nanosecond spark discharge between two-point electrodes at atmospheric pressure and at an initial temperature of 1000 K. As a first step, it was assumed that thermal and chemical effects of the nanosecond spark discharge are the same in a lean H-2-air mixture as in air. Comparing different models for the gas heating, we have shown that the fraction of the discharge energy going to gas heating for nanosecond spark discharges is in the range 20-30%, in agreement with literature values. The gas heating was found to start during the nanosecond voltage pulse as soon as the nanosecond spark channel is formed and the discharge energy is deposited non-uniformly in air with hot spots close to electrode tips. For the chemical effect, results show that in air, the atomic oxygen radical is mostly produced after the nanosecond voltage pulse due to the dissociative quenching of electronically excited N-2. The maximum of the dissociation level of molecular oxygen is in the range 30%-60% in the plasma channel. Then, thermal and chemical effects of the nanosecond spark discharge on the ignition of a lean premixed H-2-air flame have been studied separately. In both cases, a flame ignition was observed. Results show that thermal and chemical effects of the nanosecond spark on the ignition are of the same order with a slightly higher ignition efficiency for the chemical effect. Finally, we have taken into account simultaneously thermal and chemical effects and a synergistic activation has been observed. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.