This paper presents numerical and experimental results of the smoldering propagation along a thin solid in a narrow space. This study particularly focuses on the formation of a fingering pattern caused by an instability mechanism an important phenomenon from a fire safety point of view because the formation of such a pattern increases the smoldering velocity. Two-dimensional numerical simulations are first conducted to show, similarly to previous theoretical studies, that a fingering pattern including the split and local extinction of fingertip is formed only when the Lewis number is well below unity. The accuracy of the near-equidiffusive (i.e., the Lewis number near unity) approximation for such a low Lewis number is also tested. Then, the predicted influence of the Lewis number is experimentally confirmed by burning paper sheets having different Lewis numbers. It is also demonstrated (both numerically and experimentally) that a fingering pattern can be formed without convection effects. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.